Heat oven to 425° F. Sprinkle a rimmed baking sheet with the cornmeal. Shape the dough into a large oval and place on the prepared baking sheet.
In a bowl, toss the potatoes, garlic, rosemary, oil, salt, and pepper. Scatter over the dough and bake until golden brown, 25 to 30 minutes.
Toss the lettuce with the vinaigrette and serve with the flat bread.