Preheat the oven to 475°F (245° C).
Be sure you have your schiacciata or focaccia dough ready to use in the next 30 minutes (see notes for how to make potato pizza in one day without an overnight proof).
Fill a large bowl with lukewarm water and whisk in 1 ½ teaspoons of salt.
Peel the potatoes and slice them very thinly either by hand, with a mandoline slicer or the slicer on a box grater
Place the potatoes in the saltwater bath, being sure they are completely submerged and let soak for at least 30 minutes. You can also cover and refrigerate them for up to 12 hours.
Drain the sliced potatoes in a colander and pat them dry with paper towels or a clean kitchen towel, removing as much moisture as possible.
Place the potato slices in a large bowl. Add 2 tablespoons of olive oil and the chopped rosemary, if using. Gently toss, making sure all the potatoes are evenly coated with the olive oil.
Line a baking sheet (minimum dimensions 17x11.5 inches) with parchment paper and drizzle it with olive oil and spread it out evenly with your hands. Turn the schiacciata dough out onto the baking sheet. Using slightly wet hands, pull and stretch the dough out into a square (or whatever shape you can manage; it can be a little challenging to spread out), pulling from the underside of the dough outward.
Spread the potatoes in an even layer on top of the dough. Drizzle with two more tablespoons of olive oil and sprinkle with the remaining ½ teaspoon salt.
Bake for 13-17 minutes in the middle of the oven until golden brown and crunchy.
Let cool before cutting.