Heat a large skillet over medium heat and add a drizzle of oil. Add sliced onions and cook for about 15-20 minutes until caramelized.

Push onions to one side and add ground beef to the skillet. Break it apart and cook until browned, about 5-7 minutes. Drain excess grease if necessary.

Stir the onions into the beef, then add gluten-free soy sauce. If using, add mushrooms or bell peppers. Cook for an additional 3-5 minutes.

Sprinkle cheese on top and cover the skillet. Let it cook for 2-3 minutes until the cheese is melted.

Toast the buns lightly, then spoon the beef mixture onto each bun and add any additional toppings.