I am shocked! I've never heard of using a leftover baked potato to make hash browns.   No self-respecting cook or diner in the Northeast would try to use a already baked potato!!!!   Raw, raw, raw. Add those onions and perhaps some cut up green pepper. Stay away from eggs and flour, this isn't a potato pancake. Cook them in butter in a fry pan. Nothing fancy. Keep the damn sauce out of it! This isn't a swanky gourmet dish!   Douse them with some ketchup perhaps. Finest kind!