In a large bowl, combine 1 tablespoon Worcestershire sauce, 3 teaspoons mustard, and half of the onions. Crumble beef over the mixture. Mix well. Form into four patties. Grill, covered, over medium heat or broil 4� from the heat for 6 to 9 minutes on each side or until no longer pink. Meanwhile, in a small bowl, combine the cream cheese, remaining Worcestershire sauce and mustard, mushrooms, and parsley. Spread the cheese mixture on the cooked patties. Top with reserved onions. Grill or broil for 30 seconds longer or until the onions are crisp-tender. Serve on the Kaiser rolls.