Here is something to do with leftover roast beef and gravy. Boil potatoes and mash, or use instant, or mash leftover potatoes. Place layer of mashed potatoes on bottom of 2 quart casserole. Cut up roast beef and onion. Add to gravy and pour into casserole. Finish off with layer of mashed potatoes. Bake about 30 minutes at 325 degrees.