Preheat grill for high heat. In a medium bowl, combine 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 with hamburger meat. Brush hot grill with oil. Set temperature to high and cook patties for 5 min. on each side. Test to make sure they’re cooked all the way through before serving between hamburger buns with a side salad or homefries.