Blue Cheese Stuffed Hamburgers

1 1/2 lb. ground beef
1/4 tsp salt
1/4 tsp pepper
1/3 cup crumbled blue cheese
3 oz. cream cheese, softened
1 T country-style Dijon mustard
4 onion buns
1 1/2 cup sliced fresh mushrooms
2 med. onions, sliced
1/4 tsp salt
1/4 tsp pepper
1 T Worcestershire sauce
4 slices ripe tomato

In a medium bowl stir together ground beef, salt, and pepper.  Form into 8 large 1/4" patties.  In a small bowl stir together blue cheese, cream cheese and mustard.  Place about 2 T 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 minutes for medium).  Place hamburgers on buns.  In same skillet with drippings place mushrooms and onions; add salt, pepper and Worcestershire sauce.  Cook over med. high heat, stirring occasionally, until tender (4 to 5 minutes).  Meanwhile, place tomato slice on each hamburger; top with grilled mushrooms and onions.