- With vegetable slicer or sharp knife, cut potatoes into 1⁄16-inch slices. Stir together water and fine salt in large bowl until salt is dissolved. Add potatoes and let stand 15 minutes. Drain. Pat potatoes dry with paper towels. Rinse bowl and wipe dry; return potatoes to bowl.
- Preheat oven to 475°F.
- On lightly ﬂoured work surface with lightly ﬂoured rolling pin, roll dough into 12-inch round; transfer to 14½-inch pizza pan and gently stretch dough to fit pan.
- Add onion, 1 tablespoon of rosemary, 2 teaspoons of oil, and the pepper to potatoes. Toss until mixed well and potatoes are coated evenly. Arrange potatoes in concentric circles over dough, overlapping them. Scatter mushrooms over potatoes, leaving border of potatoes exposed around edge.
- Bake until potatoes are tender and golden and crust is browned along edge, about 20 minutes. Drizzle with remaining ¾ teaspoon oil and sprinkle with remaining 1 teaspoon rosemary and the coarse salt. Cut into 10 pieces.
- Per serving: 1 piece
If the pizza dough springs back when you try to stretch it, cover it with a clean kitchen towel and let it rest for about 10 minutes. This will give the gluten in the dough time to relax, making it easy to stretch.