In a skillet over a medium heat, cut bacon into 1/2 inch pieces and fry until crisp. Remove bacon and reserve drippings in skillet to fry onion and mushrooms until tender. Set aside with bacon.
In a large bowl, combine beef, pork, cheese, sauce, and seasonings. Mix together with hands and form eight thin patties that are slightly larger than the buns.
Place bacon, mushrooms, onions on four of the patties. Top with remaining patties and seal edges. Cook burgers on the grill for 5-8 minutes on each side. Do not flip more than once to prevent the burger from falling apart. Add cheese to burgers with a minute of grill time left.