Drain tuna and chop coarsely. Combine tuna, onion, mayonnaise, egg, lemon juice, salt, pepper and 3/4 cup bread crumbs in bowl. Mix well. Divide into 3 portions. Roll each portion in remaining bread crumbs and form into 3 patties. Refrigerate 1 hour.
Heat butter and oil in skillet over medium heat, cook cakes 3 to 4 minutes on each side until golden brown. Serve with tartar sauce.