Preheat Grill For High Heat. In A Medium Bowl, Mix Together Potato, Cheese, And Mushrooms. In A Large Bowl, Mix Ground Beef With Onion Soup Mix. Form Into 6 Large Burger Patties. Make A Pocket In Each Burger, Stuff With Potato Mixture, And Seal. When Ready To Grill, Brush Grate With Oil. Cook Burgers Over High Heat For 5 Minutes On Each Side, Or Until Well Done. Serve Hot.