Carmalize onions with 1 Tbs butter. Add salt and pepper.
Sauté onions with 1 Tbs butter. Add salt and pepper.
In a large bowl combine ground beef, carmalized onions, sautéd mushrooms, and egg. Be gentle and fold ingredients together. Be careful not to over mix.
Form meat into egg sized balls. Flatten balls into patties and leave a divot in the center.
Grill burgers at 600F, flipping once. Don't press down on the patties. Add cheese 1-2 minutes before taking it off the grill.
Slice open bun and butter. Add garlic if desired. Broil bun until golden brown.
Place burger on bun with tomato and lettuce.