Use leftover mashed potatoes as the topping for a shepherd's pie or cottage pie. Layer cooked ground beef or lamb with vegetables and top with mashed potatoes, then bake until golden brown. Form leftover mashed potatoes into patties and fry them until golden brown. Serve as a side dish or as a vegetarian main course. Mix mashed potatoes with sour cream, shredded cheese, and chopped herbs, then bake until bubbly and golden brown. Top mashed potatoes with bacon, chives, shredded cheese, and sour cream for a loaded baked potato-style dish.