In a saucepan, combine water, milk, butter and salt and bring to a boil, then stir in mashed potato flakes and set aside
In a large bowl, combine egg, oats, celery, onion and pepper and mix, then add ground beef and mix well
Place beef mixture on a piece of wax paper and spread out to form a rectangle
Spread mashed potatoes over beef and sprinkle with cheese
Roll up and secure the seam
Place seam side up in a greased 13 x 9-inch baking dish
Bake at 350 degrees for 30 minutes, pour ketchup over the top and cook another 20 minutes or until meat is fully cooked
