Shape 4 patties out of the ground beef. Season both sides with salt, black pepper, and garlic powder. Lightly grease an outdoor grill or indoor grill pan. Heat to medium-high heat. Add the burger patties and cook turning once until fully cooked (the internal temperature has reached 160 degrees). During the fast few minutes of cooking top each patty with a slice of Swiss cheese.
While burgers are cooking melt butter in a sauté pan over medium-high. Add mushrooms and sauté for 15-20 minutes or until tender. In a small saucepan, heat BBQ sauce.
Split hamburger buns in half. Place each hamburger patty on the bottom half of the bun. Top with mushrooms, French fried onions, and BBQ sauce. Place the top half of the bun over the BBQ sauce.