Prepare A Grill For Medium Heat. Lightly Oil The Grate. Divide The Ground Beef Into 6 Patties. Season With Meat Tenderizer, Salt And Pepper. Set Aside. Melt The Butter In A Skillet Over Medium Heat. Add The Mushrooms And Soy Sauce; Cook And Stir Until Browned. Set Aside And Keep Warm. Grill The Patties For About 6 Minutes Per Side, Or Until Cooked Through. Divide The Mushroom Mixture Evenly Amongst The Burgers And Top Each One With A Slice Of Swiss Cheese. Cover The Grill For About 1 Minute To Allow The Cheese To Melt. Remove From The Grill And Serve On Hamburger Buns.