Potato and Rosemary Pizza Rustica

Grease a large bowl with 1 tbsp. of the oil and set aside. Put flour, yeast, sugar, and 1⁄2 tsp. salt into the bowl of a standing mixer fitted with a dough hook and mix on lowest speed until well combined. Increase speed to medium-low and gradually drizzle in 1I cups cold water, mixing until a dough begins to form, about 1 minute. Stop mixer and scrape down sides of bowl with a rubber spatula. Continue mixing on medium-high speed until dough is smooth and elastic and cleanly pulls away from sides of bowl, 10-12 minutes more. Shape dough into a ball, transfer to oiled bowl, and roll dough around in bowl to coat all over with a thin layer of oil. Cover bowl with a clean damp dish towel and set aside in a warm spot to let dough rise until doubled in bulk, about 2 hours.

Meanwhile, peel and very thinly slice potatoes on a mandoline or with a very sharp knife, transferring them to a large bowl of ice water as sliced. Soak potatoes for 30 minutes to remove excess starch, then drain in a colander. Repeat soaking and draining process 2 more times. Set colander over a large bowl. Toss potatoes with 1⁄2 tsp. salt and set aside to let drain for 10 minutes. Transfer potatoes to a medium bowl. Add onions and 1 tbsp. of the oil, toss well, and set aside.

Preheat oven to 475°. Grease 2 large (about 13" x 17") cookie sheets with 2 tbsp. of the oil each and set aside. Transfer dough to a lightly floured surface, divide in half, and shape each into a rectangle. Transfer each piece of dough onto a cookie sheet. Using your hands, gently flatten and spread dough out to edges of cookie sheet into a very thin rectangle. (Dough will shrink back a bit each time you spread it out, so let dough rest or "relax" for 10-15 minutes each time after you've spread the dough out as far as it will go.) Brush each dough with 2 tbsp. of the oil. Evenly spread half the potato-onion mixture over surface of each dough, leaving a 1" border around edges. Season to taste with salt, drizzle remaining oil over each, and sprinkle each with half the rosemary.

Bake pizzas until crust is crisp and dark golden brown around the edges and potatoes are ringed golden brown, about 20 minutes, rotating cookie sheets halfway through baking. Transfer pizzas to a cutting board and serve hot or at room temperature.