Dissolve one tablespoon salt in a large bowl with warm water. Slice the potatoes super thin (1/16th inch), ideally on a mandolin (but be careful those things are sharp!). Soak the potatoes in the water while you prepare the pizza dough. (30 min soak or up to 12 hours in the fridge).
Preheat your oven to 500 degrees.
Drain the potatoes and dry on a clean kitchen towel. It is important that they are nice and dry so put them in a single later. Place the potatoes in a bowl and add two tablespoons of olive oil and the minced onion. Toss to coat.
Oil a rimmed cookie pan (13 x 18) with a teaspoon or two of olive oil. Roll dough out to on a lightly starched surface or on oiled parchment paper for easier transfer to roughly fit the pan. Place the dough in the oiled pan and using your fingers or a roller, press the dough so it covers the entire pan – it will be thin.
Drizzle the dough with an additional teaspoon of olive oil. Top with potatoes. Sprinkle with the remaining ½ teaspoon salt, pepper and the rosemary leaves. Drizzle with the remaining olive oil.
Bake for 25 to 30 min, until the topping is potatoes are starting to turn golden brown and the crust is nicely bronzed underneath. Serve hot or at room temperature.
