Finely chop the onion and mushrooms.
Add onion, mushrooms, salt, pepper, and egg to the ground beef and mix well.
Shape the mixture into round patties.
Heat olive oil in a pan and grill the hamburger steaks on both sides.
Top the grilled hamburger steaks with mozzarella cheese and melt.
Finish by drizzling tomato sauce or a low-carb sauce.