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2 tablespoons olive oil
1/2 yellow onion, thickly sliced
8 mushrooms, thickly sliced
1 tablespoon butter
salt and pepper
2 pounds lean ground beef
4 tablespoons soy sauce
1 tablespoon balsamic vinegar
1 tablespoon Worcestershire sauce
3 cloves garlic, minced
fresh ground pepper
1. In a skillet over medium-high heat, saute onions in olive oil. Lower heat lightly once onions start to brown and cook for about 15 minutes.
2. Raise heat back to medium-high, then add butter and mushrooms, cook another 10-15 minutes until mushrooms are soft and browned. Season with salt and pepper.
3. Gently mix beef with soy sauce, balsamic, Worcestershire, garlic and pepper. Form into patties and grill or pan fry.
4. Toast bread, then spread with butter and mayonnaise. Top with beef, onions and mushrooms.