Warm up the leftover mashed potatoes just until they are soft enough to stir. Add a spoonful or two of sour cream (to your taste). Add about 2 Tablespoons melted butter, salt, pepper, and shredded cheddar cheese. Stir all together until well blended
Pour into oven safe casserole dish. Bake at 350° for about 10 minutes, until cheese has melted and potatoes are warmed through completely.