How to Make It
On a lightly floured work surface, roll or stretch each piece of levain dough to a 6-by-12-inch rectangle. Lightly brush the dough with olive oil, cover it loosely with plastic wrap and let stand at room temperature until slightly puffed, about 45 minutes.
Meanwhile, put a pizza stone on the bottom of the oven and preheat the oven to 500°, allowing at least 30 minutes for the stone to preheat.
Work with 1 dough rectangle at a time. If the dough has shrunk, gently stretch it to a 6-by-12-inch rectangle and transfer it to a floured pizza peel. Spread 2 tablespoons of the crème fraîche on the dough and arrange half of the potato slices on top, overlapping them slightly. Sprinkle half of the rosemary and lemon zest over the potatoes and drizzle with 1 tablespoon of olive oil. Season with Maldon sea salt and black pepper.
Slide the rectangle onto the hot pizza stone and bake for 8 to 10 minutes, until golden brown and crisp. Slide the tarte flambée onto a work surface, cut into 2-inch strips and serve. Repeat with the remaining dough and toppings.
You can buy raw levain dough from select bakeries. Maldon sea salt has a great crunch and a remarkably subtle, briny flavor; it is available at most grocery stores and online.