Preheat grill to high heat. I prefer this on the grill but a griddle or a frying pan works.
Mix beef, bread crumbs, egg, and mozzarella. Use your hands, mix thoroughly. Adding steak sauce is optional and subject to a lot of variations. I have done this with A-1, Heinz 57, and Country Bob's Steak Sauce. All worked. I actually liked the Country Bob's Steak Sauce quite a lot. Available from Wal-Mart's everywhere.
Cook patties 5 minutes per side, longer if you prefer well-done. Baste with additional steak sauce if desired.
Serve with a slice of onion and some pickle. It's a burger, dress it as you like!