Fry meat with onion and salt and pepper to taste. When meat is finished cooking, add liquid from cans of peas and carrots and corn. Simmer for 5 minutes. Remove meat from liquids. Make a thick gravy by mixing flour and water. Return meat and mix well. Pour into casserole dish. Boil potatoes in water with salt. Drain and mash potatoes with margarine. Add milk and beat potatoes until smooth. (Add more milk to make potatoes creamier.) Add peas and carrots in a layer on top of meat, followed by a layer of corn. Smooth potatoes on top. Beat egg and brush over potatoes. Add grated cheese. Cook uncovered at 350°F until potatoes are browned.