Score flank steak on both sides. Marinate steak in Italian dressing for 8 hours or overnight in covered container. Remove steak from marinade.
Pat dry. Broil steak 6 to 8 minutes on one side. Turnover. Broil 5 to 6 minutes on the other side. Mix bleu cheese, onion and butter or margarine together to form paste.
Put a thick layer on top of steak. Broil until cheese mixture melts. Cut into thin slices. Serve.
Servings: 4