Left over mashed potatoes??? What is that????
I have made potato pancakes (that was before I quite peeling them), add self rising flour or equavalent and egg, fry in skillet in butter.
I have also used them to make a casserole we like which has potato topping with grated cheese on top. (I brown ground beef, onion, garlic, season with salt and pepper. Drain, add drained canned green beans, dry basil and tomato soup, mix, place in greased casserole, put 'taters on top and bake until bubbly and cheese is melted.
I have never frozen them. They do last a few days in the fridge and reheat nicely in the micro wave or add a little milk and heat on top of stove, low heat, stir often.