04Heat 1 tablespoon olive oil in a large skillet set over medium heat. Add onion slices, cover and cook until onion is golden brown, stirring occasionally, about 20 minutes.
05Top the dough with tomato slices, caramelized onions, rosemary, Parmesan cheese, and the remaining salt. Drizzle with the remaining 2 tablespoons olive oil.
06Bake until the focaccia is golden brown, about 20 minutes. Remove from the oven and allow to cool on a rack. Cut into pieces and serve.