Potato Tarte Flambée
Ingredients:All-purpose flour, for dustingTwo 6-ounce balls of levain dough (sourdough; see Note)Extra-virgin olive oil, for brushing and drizzling1/4 cup crème fraîche1/4 pound Red Norland or other red potatoes, sliced crosswise 1/16 inch thick on a mandoline1/4 cup lightly packed rosemary leaves2 teaspoons finely grated lemon zest, preferably Meyer lemonMaldon sea salt (see Note)Freshly ground black