* Each flank steak should weigh between 1 and 1 1/2 lbs. Place beef steaks in shallow glass or plastic dish. Mix remaining ingredients and pour over the beef. Cover and refrigerate at least 8 hours, turning occasionally. Set oven control to broil. Place beef on rack in broiler pan. Broil with the tops about 3 inches from the heat until brown, about 5 minutes. Turn beef and broil about 5 minutes longer. Cut beef in thin strips across the grain of the meat. Serve with tortillas and guacamole if desired.