Preheat the oven to 375 degrees F.
Place the 2 bread halves on a large baking sheet. Spread the tomato sauce evenly over both halves and top each with the Mozzarella cheese. Arrange the meatballs over the cheese and top with sliced onions, red pepper flakes and mozzarella cheese. Bake until the cheese is bubby, the bread is toasted, and the onions are wilted and start to color, about 20 minutes.
Yield: 2 to 4 servings