Place each piece of tenderloin between 2 pieces of plastic wrap. Flatten slightly with heel of hand. Sprinkle surfaces of pork with lemon pepper. Heat butter in heavy nonstick skillet; brown pork evenly, about 3-4 minutes on each side. Remove to serving platter, keep warm. Add lemon juice, Worcestershire sauce and mustard to skillet. Cook, stirring with pan juices, until heated through. Pour sauce over medallions, sprinkle with parsley and serve.