In medium skillet, sauté mushrooms, onion, and garlic in butter until tender, about 2 minutes. Combine ground pork, mustard, Worcestershire sauce, salt, and pepper; mix well. Shape into 12 patties, about 4 inches in diameter. Set aside. Spoon equal portions of mushroom mixture onto center of 6 pork patties. Spread to within 1/2 inch of edge. Top with the remaining 6 patties; seal edges. Place patties on grill about 6 inches over medium-hot coals. Grill 10–15 minutes, turning once. Serve on buns, if desired. Serves 6.