Instructions

1. Prepare the porchetta: On a rimmed baking sheet, lay the pork belly skin side up and rub the skin side generously with salt. Refrigerate uncovered at least 12 hours or ideally 24 hours.
2. Meanwhile, in a small skillet over medium heat, add the fennel seeds. Cook, stirring or shaking the pan occasionally, until lightly toasted and fragrant, 4-5 minutes. Remove and coarsely grind.
3. Set a rack in the center of the oven and preheat to 300°. Retrieve the pork belly and place skin side down on a clean work surface. Rub the inside all over with salt and pepper, then with the fennel seeds, fennel pollen, fennel fronds, parsley, sage, and rosemary. Baste or rub the loin with the white wine if using. Wrap the seasoned pork belly firmly around the loin to cover completely, then tie the porchetta tightly every 2-3 inches with butcher twine. Transfer to a roasting rack set in a high-sided roasting pan (if the belly does not fit around the entire loin, be sure the largest piece of the belly is facing the top of the oven).
4. Roast until the skin looks firm and the ends of pork look cooked, 2 hours 15 minutes. Raise the heat to 425° and cook, basting the skin once with the drippings halfway through, until the skin is well browned and a thermometer inserted into the center of the loin registers 145-150°, about 1 hour.
5. Meanwhile, make the marinated onions: In a large bowl, combine the onions and vinegar. Let rest, stirring occasionally, until the onions soften, 1 hour or up to overnight. When ready to serve, stir in the olive oil.
6. Make the salsa verde: In a large bowl, stir the bread crumbs and red wine vinegar. Add the parsley, olive oil, capers, anchovies, eggs (if using), and garlic, and stir to combine.
7. Remove the porchetta and let rest 15 minutes before slicing into thin (1⁄2- to 1-inch-thick) slices.
8. Spread some of the salsa verde onto the bottom half of each sandwich roll. Top with the porchetta slices and marinated onions to taste.