For two 13 x 9-inch pans, or one half-sheet pan

3/4 C. plus 2 T. (130 grams) unbleached bread flour
3/4 C. plus 2 T. (130 grams) unbleached all-purpose flour
1/4 tsp. instant yeast
1 1/4 cups (285 grams) lukewarm water
1/2 tsp. granulated sugar
1/2 tsp. salt
Extra-virgin olive oil.

Mix the flours and yeast in a mixing bowl. Using the paddle attachment, add water on low speed until the batter comes together. Increase the speed to medium and beat for 20-25 minutes until it cleans the bowl and comes together. (If it hasn’t started to come together in 20 minutes, add a few pinches more flour). The dough should still be quite wet. Add sugar and salt and mix for another 2-3 minutes. Cover the dough with plastic wrap and let it rise about 4 hours. Coat the baking pan or pans with olive oil. Pour the dough directly into the pan. With hands coated with olive oil, press the dough into a thin layer. Let it rest for 10-15 minutes, and then finish pressing it into the whole pan. It will be quite thin. Cover and let rise for another hour. Meanwhile, slice 7 Yukon Gold potatoes, peeled (about 4 lb.) into thin slices. Toss them with salt and let them exude moisture for about 15 minutes. Squeeze them dry in a colander to release more of their liquid. Toss them with a thinly sliced onion and about 2 T. chopped fresh rosemary. About 30 minutes before baking, preheat the oven to 425 degrees F (220 C) and put one rack on the oven’s top shelf. If using two pans, put another rack on the lower third. After the dough is proofed, spread on the potato topping. Brush with olive oil and sprinkle with more salt. Bake until the potatoes are brown at the edges and easily pierced with the tip of a knife, about 40 minutes.