In medium bowl stir together ground beef, 1/4 tsp salt and 1/4 tsp pepper. Form into 8 large 1/4" thick patties. In small bowl stir together blue cheese, cream cheese and mustard. Place about 2 tbsp cheese mixture on top of each of 4 patties. Top each with remaining meat patty. Press around edges to seal. Place hamburgers in 10" skillet. Cook over med heat, turning once, until desired doneness (12-15 min for medium). Place hamburgers on buns. In same skillet with drippings place mushrooms and onions; add 1/4 tsp salt, 1/4 tsp pepper and Worcestershire sauce. Cook over med high heat, stirring occasionally, until tender (4-5 min). Meanwhile, place tomato slice on each hamburger; top with grilled mushrooms and onions.