In skillet, sauté mushrooms, onion and garlic in butter until tender, about 2 minutes; set aside.
Combine ground pork, mustard, Worcestershire sauce, salt and pepper; mix well. Shape into 12 patties, about 4 inches in diameter. Spoon mushroom mixture onto center of 6 patties. Spread to within 1/2 inch of edges. Top with remaining 6 patties; seal edges.
Place patties on grill about 6 inches over medium coals. Grill 10 to 15 minutes or until no longer pink in center, turning once. Serve on buns, if desired.