You poke your head in and ask, hopefully, for a table for four. It’s an easy blunder to make if you’re not from around here.

"Disculpa, privado," says a man seated at a table near the entrance. "Sorry, private." There's no table for you here—not tonight or any night. Privado.

The boisterous Basque restaurant of your dreams isn't a restaurant at all, but one of San Sebastián's fabled sociedades gastronómicas, members-only social clubs whose existence revolves entirely around food. In a town where cooking and eating seem to be the raisons d'être for just about everything, from the three-star restaurants to the napkins-on-the-floor pintxo joints, these culinary clubs, which have been around for about 150 years, still harbor some of the most interesting kitchens of all.

“The sociedades are basically unknown to foreigners, but they’re so central to Basque culture and cuisine,” says Fernando Barcena, the longtime right hand of Juan Mari Arzak, chef-patron of his celebrated Arzak restaurant. “Almost all of the famous Basque chefs belong to sociedades, and lots of the regular members are great chefs themselves—they just don’t work in restaurants.”

For outsiders, the challenge is getting into these places, since you can enter them only with a member. Fortunately I once spent a year in San Sebastián and still remember very long meals at two sociedades, where I was seduced by their unique brand of homey, epicurean Basqueness. So when I returned recently with the goal of hanging out in a few more of them—this time with a notebook and camera in hand—there were some people I could call. Which is not to say the invitations came easily: Multiple WhatsApp messages had to be exchanged, introductions had to be made in the café upstairs from the fish market, the word privado was repeated a few more times. Finally, on a Wednesday at 7 p.m., I walked unimpeded through the door of La Unión Artesana, San Sebastián's oldest sociedad, to meet Angel Soret, who joined back in 1963, when he was 17 years old. Soret, a bald-headed former ham salesman with a crooked smile, brings to mind Dick Cheney's wisecracking Basque cousin. At the club he's known for his merluza en salsa verde, a traditional dish of hake with clams in a parsley-based green sauce, which he'll be making tonight for me and five others.

As Soret selects utensils and pans from the steel shelves in the kitchen, which looks like one you'd find in a smallish modern restaurant (eight burners, lots of good knives), he explains how things work at La Unión Artesana. Each of the club's 270 members pays an initiation fee of around 3,000 euros, plus yearly dues, which gets him a key to the front door. (And it is usually a him, though the traditionally all-male clubs have been gradually going coed.) Members can come at any time of day and cook with as many guests as they want; they bring their own meat and fish and vegetables but can use the olive oil, eggs, sugar, and other basics stocked in the communal pantry. There are also unlocked cabinets from which everyone can help themselves to wine or cider or mini bottles of every imaginable kind of liquor, before leaving cash for it at the end of the night.

Miraculously, nobody cheats. “When I invite friends here from other places, even from other parts of Spain, they can’t believe that we operate on the honor system,” says David Vega, another club member who has joined us for the meal. “They say, ‘If we tried this in Madrid, the cabinets would be emptied on the first night—we would just grab all the bottles and run home.’ ” This leads me to wonder whether Basque people are more trustworthy than other people. It’s less a question of honesty than one of culture and tradition, Soret says. “Members see this place as their home, and you don’t steal from your own home.”

While chopped garlic and parsley are simmering in olive oil inside a rondeau pan, Soret unwraps about a pound of raw kokotxas that he bought with the hake at the central La Bretxa fish market. Shiny and wiggly and heart-shaped, kokotxas are fish jowls, in this case hake, cut from under the mouth; a Basque delicacy, they looked good to Soret at the market so he's adding them to the dish. With the fastidiousness of a bonsai gardener, he trims each one using shears, then sets them aside and places the fillets in the pan. For him it's sacrilege to use even a pinch of flour to thicken the salsa verde, though many restaurants and home cooks stir it in by the spoonful.

Soret prefers to periodically remove the pan from the flame and spend several minutes shaking it gently in a tight circular motion, so that the gelatin from the hake fillets slowly seeps into the sauce. Adding the parsley to the pot at the very beginning is another trick of his, which requires more care but makes the sauce a deeper color (“It’s called green sauce, so it’s nice when it’s actually green,” he explains). While Soret cooks, the rest of us do a little chopping and slicing when asked, but we mostly stand around drinking cider as he makes a stream of unprintable jokes about soccer, Donald Trump, and all the young supermodels that he suspects will flock to the Basque Country to meet him after seeing his picture in this magazine.

If you pay attention in the kitchen of a sociedad, you'll see plenty of evidence of the well-known hallmarks of Basque cuisine. There's the emphasis on the best and freshest products: Soret bought everything a few hours earlier from his two trusted merchants in the neighborhood, Sara the fishmonger and Zouhair the fruit vendor. There's the deft mixing of tradition and modernity: Soret's recipe is loosely based on his father's, but his meticulous pan-shaking technique and other crucial tweaks are his own. And there's the tendency of the cook to show off, just a bit, while not hesitating to share his knowledge with others. A guy named Iñaki who's next to us at the stove, boiling three txangurros for his own group of six, keeps eyeing Soret's technique, and Soret clearly enjoys the attention—now he's cooking the kokotxa trimmings separately over a low flame, making a kind of gelatin booster sauce that he'll add to our salsa verde. When he realizes he's got more of it than he needs, he tells Iñaki, "It's all yours," and Iñaki takes a few spoonfuls to thicken the garlic-based pil-pil sauce that his friends are preparing for their main course of cod.

One important thing that everyone mentions about the sociedades: They are democratic places where a person's wealth and status in the outside world are pretty much irrelevant. It's said that if a billionaire arrives at a club with his chauffeur, once they cross the threshold, they're equals. In fact, if the chauffeur makes an impressive marmitako (tuna soup), he's the real boss for the night. The person at the table who gets the most respect, always, is the one who's the best cook.

Tonight that would be Soret. We all agree that merluza en salsa verde should always taste like this, since the fish has that ideal firm-but-not-chewy texture and the delicate kokotxas almost bleed into the rich, silky sauce. Along with the hake we have a simple tomato salad, a platter of local anchovies, four bottles of cider and a few more of wine, followed by a round of gin and tonics. The bill comes to 30 euros per person, about $35. Vega collects the cash and puts it in an envelope, along with a checklist of everything we consumed, and slips it through a slot in the office door.

To understand the sociedades, it helps to understand a few things about the Basque people, who populate this region of emerald hills and craggy cliffs on both sides of the French-Spanish border. Over the centuries here on the Spanish side, the Basques have operated with varying degrees of autonomy from the national government in Madrid; there have been periods of repression and also rebellion, including one decades-long stretch when the armed separatist group ETA carried out frequent attacks.

But all along, ordinary Basques have continued doing their own thing. It's not just that they eat Basque foods and drink Basque wines and speak a language that has no traceable links to French or Spanish or any other tongue. (Foreigners who attempt to learn Euskara, with its complicated syntax and consonant-heavy, throat-clogging words, usually give up quickly.) The Basques also play Basque sports—handball, jai alai, tug-of-war—and give their children Basque names. Spend a week here and you'll likely meet several Arantxas and Enekos and Iñigos, and probably a Gaizka or two. And there are plenty of traditions that survive against all odds, including festivals with screaming contests where people erupt in an irrintzi—a high-pitched primal yell that sounds like a cross between a Berber ululation and a slasher-film shriek. Centuries ago, farmers and shepherds used irrintzi to communicate between far-flung pastures. "It's the Basque SMS," a man named Patxi quipped to me.

People from other parts of Spain will tell you that the Basques are some of the most decent and likable people you could meet, and also the most clannish and mysterious. All of this is true. If you move to San Sebastián from somewhere else, you'll probably never be part of a cuadrilla—the local term for your gang of childhood friends, who remain your closest friends until death—which means you will never be truly integrated into Basque life. Cuadrillas, of course, love to hang out in sociedades, often the same ones their parents loved to hang out in.

One afternoon a friend of a friend introduces me to Luis Mokoroa, the president of La Cofradía Vasca de Gastronomía, a club high on a hill overlooking San Sebastián's old town. Mokoroa, whose muttonchop sideburns are worthy of a Civil War general, uncorks a bottle of fizzy local txakoli and starts to show me around. This club is actually both a sociedad and a cofradía—an association founded in 1961 to protect and promote Basque cuisine, with its own culinary library and schedule of events open to the public.

But as at many of the older sociedades, the atmosphere is part Elks Lodge, part rustic Pyrenees tavern, with beamed ceilings and paneled walls adorned with every manner of cooking award and faded group photo, plus a framed list of renowned honorary members (Elena Arzak, Ferran Adrià, Martín Berasategui). Pointing to a glass display case of antique military uniforms, Mokoroa explains how it came to be that every year on January 20, all of the local sociedades send out squadrons of marchers for the traditional Tamborrada drum parade, the city's biggest and probably strangest holiday. Dating back to the days of the French occupation, when much of the city was destroyed in a fire, it's now a 24-hour reenactment bacchanal where some groups wear Napoleonic army outfits, others dress in chef's whites, and all walk through the streets banging drums or barrels strapped around their necks. (On this day, sociedades open their doors to everyone.)

Since Mokoroa has friends at practically every sociedad, he offers to walk me around to a few whose doors are open. Just down the hill is the venerated Gaztelubide, where a mixed group of 40 or so is getting ready for a lunch of bacalao a la vizcaina, a classic cod dish made with local choricero peppers. Vintage photos on the walls of the long, narrow room show some of the raucous Tamborrada celebrations of the past. One night in the 1950s, when this place was still an all-male stronghold, famed Italian soprano Mafalda Favero disguised herself as a man to make it past the door, thus becoming the first woman ever to have dinner at Gaztelubide.

And what's up with the for-men-only thing in 2017? When it comes to gender issues, once again the Basques have their own set of norms. For centuries there was a strong matriarchal tradition here, partly because women ran the households while their husbands spent long periods away on the ocean or in the fields, fishing or herding animals. The all-male sociedades began forming around 1870—no one is sure exactly why, but it probably had to do with the severe restrictions on tavern opening hours in San Sebastián at the time—and it's said that women not only tolerated the clubs but encouraged them. "Some think it was a way for them to control us," says Barcena. "They knew exactly where we were, who we were with. And that what we were doing was relatively harmless."

In the past few decades many of San Sebastián's approximately 120 sociedades began opening their doors to women—first only as guests, then as full-fledged members. The evolution happened slowly, and it's still happening. A few of the oldest clubs remain all-male, and even in some where women can enter freely as guests, there's an unwritten rule that they can't enter the kitchen. "It's complicated," Vega tells me. "A lot of older members want to maintain the traditions, while some guys in their 20s and 30s won't join if they can't bring their wives or girlfriends."

Several young, progressive-minded Basque women tell me they have no problems with sociedades that don't allow them to be members or to cook. "We grew up with it, and it's a nice role reversal," says social worker Amaia Añua. "When you eat there, you're sitting at the table enjoying yourself, and the men are the ones serving you and doing all the work—and they're actually happy doing it." Other women consider the male-dominated sociedades absurdly antiquated, so they avoid them in favor of the fully coed clubs.

While making the rounds with Mokoroa, I met a San Sebastián native named Gorka Arbaizagoitia, who invited me to join him the next day at one of San Sebastián's most old-school sociedades, Gaztelupe. In the dining room, medieval-style iron chandeliers cast a dim glow over a century's worth of antique banners and coats of arms, while the kitchen is a bright zone of sparkling steel appliances, stacks of roasting pans, and a recycling bin labeled paper and cardboard. For lunch Gorka broils a top-notch chuleta—one of those dinosaur-size Basque rib steaks that shows the magic that can happen when good beef meets high heat and nothing else but rock salt. In classic fashion the outside is crisply charred and the center is a deep crimson, just barely warm.

Many sociedades including Gaztelupe have long waiting lists for membership. And even though several people admit to me that the clubs have been a bit slow to adapt to the times, most put their chances of disappearing at approximately zero. One night at 9 p.m., thanks to a referral from famed Spanish chef José Andrés, I show up at a basement-level club called Artzak-Ortzeok, where Iñaki Uranga, who runs the place, is expecting me.

From the street I can smell the signature sociedad aroma of cigarette smoke and sizzling chuleta fat. Inside, at a single long table, about two dozen people, most in their 30s, are celebrating after a boxing match. The raucous group includes both boxers, one of their moms, a visiting Qatari, and two women from Madrid. As one guy opens a bottle of Petritegi—the cloudy, funky local cider—he explains why everyone pours it while holding the bottle overhead, five feet above the glass. "Because it aerates the cider and adds bubbles," he says. "And because it's fun." One of the women inches up to the kitchen and pokes her toe over the line, joking that there's probably an invisible electric fence to keep her out (there isn't).

At another table, finishing their steaks, are 82-year-old Pedro Amaima and 86-year-old Javier Deran, both from the neighborhood. How long have they been coming to Artzak-Ortzeok? "For about 50 years, every Saturday night," Pedro tells me. "In the old days our group was much bigger, but at a certain point people started to, you know, disappear." After they clear their plates and light cigars and open mini bottles of brandy, it's time for a few rounds of dominoes and a card game called txintxon.

Some nights, instead of steak, Pedro and Javier might prepare another dish, like a bacalao al pil-pil. What doesn't change, Pedro tells me, is the joy they get from their ritual of cooking and eating and drinking and playing txintxon at their sociedad. The routine was the same last week, he says, and will be the same next week, and the week after that.