Brown beef in skillet with onion, salt and pepper.
Cook the sliced potatoes in water with 1 tsp. salt--after they start to boil cook them for 5 minutes. When beef is browned, add mushroom soup to the beef and stir well.
Grease a 2 qt. casserole. Put beef/soup mixture in the casserole. Drain water from potatoes. Add potatoes to the casserole. Stir everything with a spoon. Sprinkle cheddar cheese over the top of the casserole. Bake at 350 degrees for 30 minutes. Before serving, add sour cream to the top of the casserole.