Prepare a grill for medium heat. Lightly oil the grilling surface.
Season the ground beef with meat tenderizer, salt and pepper. Set aside.
In a medium saucepan over medium heat, melt the butter. Add the mushrooms and soy sauce, cook and stir until browned, about 5 minutes. Set aside to keep warm.
Grill the patties for about 6 minutes on each side, or until they are fully cooked. Place a slice of Swiss cheese on top of each burger, then spoon over the mushroom mixture. Grill for about 1 minute to allow the cheese to melt. Serve on hamburger buns immediately.
