In a skillet, brown ground beef and onion, drain well. Stir in soups, beans and mushroom. Simmer until heated through. Meanwhile, combine hot mashed potatoes, egg, salt and pepper; mix well. Spread into the bottom and up the sides of a greased 13-in.x9-in.x2-in. baking dish or two greased 9-in. pie plates. Spoon ground beef into the potato crust. Bake at 350 degrees for 45-50 minutes or until lightly browned. Remove from oven; sprinkle with french-fried onions and cheese. Return to oven for 5 minutes or until cheese is melted.
Note: If freezing don't include cheddar cheese or french-fried onions prior to freezing.