1. Mix the flour, yeast, salt, and sugar in a medium bowl. Add 2/3 cup/150g of tepid water and stir until smooth. If the dough feels very dry and isn’t coming together smoothly, mix in a scant tablespoon of water. Cover the bowl with a kitchen towel and let rise until doubled in volume, about 2 hours.
2. To prepare the potatoes, mix the 1 quart of tepid water with the salt in a large bowl, stirring to dissolve the salt. Use a mandoline or a chef’s knife to slice them as thinly as possible. As you cut them, put them into the salted water so they don’t discolor. Let soak for at least an hour at room temperature. (They can be done up to 12 hours in advance, and refrigerated.)
3. When ready to bake the pizza, preheat the oven to 475º or 500ºF (245º – 260ºC), depending on how high your oven will go. Put the rack in the center of the oven. Lightly oil a baking sheet that’s approximately 13- by 18 inches (33 x 46cm.)
4. On a lightly-floured countertop, roll the dough into a ball. Cover with a damp towel and let rest 30 minutes.
5. Put the pizza dough on the center of the baking sheet, floured side up, and stretch it lengthwise down the center of the baking sheet, using your hands to coax it in both directions. Then use your hands to stretch the dough towards the edges of the pan. If the dough is too stretchy and elastic, let it rest for 5 minutes, and continue. The dough will likely tear in places; simply pinch those parts back together and continue. Stretch the dough until it’s about 1 inch (3cm) away from the edges of the pan. (You’ll notice that in the picture of the post, I stretched the first one that I tested all the way to the edges but found I liked the dough a little thicker, so the second time, I didn’t stretch it as far.)
6. Drain the potatoes well and squeeze out as much water as you can without breaking the slices. Put the slices in a bowl and mix them with the diced onion, plenty of freshly ground black pepper, and the olive oil.
7. Spread the potato mixture over the top of the pizza dough, making the layer of potatoes closer to the edges a little thicker. Sprinkle the rosemary on top and bake the pizza until the potatoes start to brown and the crust is golden brown, about 25 to 30 minutes, depending on the heat of your oven. But check before the recommended baking times as this pizza is best if it’s not overly crisp.