For an extra layer of flavor, consider adding a pinch of red pepper flakes to the sautéing onions. Feel free to substitute the mozzarella with another cheese of your choice, such as cheddar or Gruyère. If you don't have leftover mashed potatoes, you can quickly microwave a fresh potato and mash it with a little butter and milk. Experiment with different types of salsa to customize the flavor profile of the dish.