Combine the ground beef with 1 tablespoon Worcestershire sauce, 3 teaspoons mustard, and half the onions. Form into four patties and broil or grill to desired doneness.
In a small bowl, combine cream cheese, remaining Worcestershire sauce and mustard, mushrooms, and parsley; mix well. Spread cheese mixture on cooked patties, top with remaining onions, and broil or grill 30 seconds more or until the onions are golden. Serve on Kaiser rolls.