One hour before you want to eat, preheat the oven to its hottest setting, 500-550°F. Dust the dough generously with flour and place on a floured work surface. Gently shape dough into a 9×13″ rectangle. Arrange dough on a floured baking sheet.
Brush olive oil all over the dough and spread the mashed roasted garlic. Layer the potatoes, lightly overlapping. Add the onions and fresh rosemary. Finish with a light sprinkle of salt and freshly ground pepper.
Bake the pizza until the bottom of the crust is crisp and the top is blistered, about 10-12 minutes. Transfer to a work surface to slice. Serve with crushed red pepper flakes and enjoy hot.
