Twice Baked Potato

Cold leftover mashed potatoes (enough to fill baking dish)
1 medium onion, chopped
1/8 -1/4 cup milk
1 cup shredded cheddar cheese

Mix the chopped onion into the cold mashed potatoes. Add milk a little at a time, if the potatoes are too stiff and/or thick. Put potato mixture into a greased 9 x13 baking dish and spread/mash evenly into the dish. Sprinkle shredded cheese over top of potato mixture.  Cover dish with foil or a lid and bake in a 350 degree oven for 30 minutes.