Combine flour and yeast into a mixing bowl of a standing mixer. Using the paddle attachment, mix on low while slowly adding the water. Beat for 3 minutes, until combined and then increase speed and beat until a very wet dough forms, about 20 minutes (see photo) Add sugar and salt and mix for another 3 minutes until dissolved.
Cover the dough and let rise for about 4 hours, until it is very light. Then, lightly coat a 1/2 sheet pan or two 9x12 pans with olive oil, and place dough on pan. Coat hands with olive oil and spread dough. Let dough rest for a few minutes every few minutes if it is difficult to spread. Cover the pan and let rise for about 1 hour until is it risen by half.
While the dough is rising, peel and slice the potatoes using a mandolin or the 2-millimeter blade of your food processor. Toss the potatoes with a sprinkling of salt and let them exude their liquid for about 15 minutes. Gently squeeze them dry and them toss with onion and rosemary
About 30 minutes before baking, preheat the oven to 425 degrees F, and arrange a rack on the top shelf for the half sheet pan or top and lower third shelves for 2 pans.
When the pizza is proofed, top with the potato mixture and brush with some olive oil. Sprinkle with more salt, and bake until pizza and potatoes are brown and crusty, about 40 minutes. Serve hot or cold