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, 1 teaspoon salt, and ¼ teaspoon pepper and toss. Set aside.
Wipe out the skillet, turn it upside down, and sprinkle the bottom with cornmeal.
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. Top with the potato mixture, arranged evenly, leaving a 1-inch border.
Transfer the skillet to oven and bake until the crust is golden brown, about 20 minutes. Slice the flat bread into wedges.