Don’t toss out leftover mashed potatoes. Add them to soups, stews, pancakes, bread, biscuits, meatloaf, meatballs, or make gnocchi. You can make potato patties from leftover mashed potatoes, too. Simply add an egg to 2 cups mashed potatoes, form into patties and fry in a pan with a little oil. You can add meat, seasoned breadcrumbs, cheese or onion for extra flavor. The first reader tip shares another way to use up leftover mashed potatoes.