Heat oven to 450° F.
Heat the oil in a large cast-iron skillet over medium heat. Add the onion and cook until golden, 5 to 7 minutes. Transfer the onion to a bowl. Add the potato, rosemary, salt, and pepper and toss; set aside.
Wipe out skillet, turn it upside down, and sprinkle the bottom with the cornmeal. Place the dough on a work surface. With your hands or a rolling pin, gently shape the dough into a round the same size as the skillet. Place the dough on the bottom of the skillet. Arrange the potato mixture evenly over the dough, leaving a 1-inch border. Bake until the crust is golden brown, about 20 minutes. Slice into wedges.