In a bowl, combine potatoes and egg. Using an ice cream scoop, place eight mounds of potato mixture onto a greased 15x10x1-in. baking pan.
Combine the onion, celery, beef, oats, milk, salt and pepper; shape into eight meatballs. Press one meatball halfway into each potato "cup". Bake at 350° for 35-40 minutes or until the meat and potatoes are browned.
Meanwhile, for gravy, melt butter in a small saucepan; remove from the heat. Blend in flour. Stir in water and bouillon. Add browning sauce if desired. Cook and stir until smooth and thickened. Spoon gravy over potato cups and pass any remaining gravy.