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PREPARE salad dressing as directed on envelope, using the red wine vinegar. Add mustard; mix well.
SCORE both sides of steak; place in resealable plastic bag. Add 1/2 cup of the dressing mixture; seal bag. Turn to evenly coat steak with dressing mixture. Refrigerate 2 hours to marinate.
PREHEAT broiler. Remove steak from marinade; discard bag and marinade. Place steak on rack of broiler pan. Broil, 6 inches from heat, 8 to 10 min. on each side or until medium doneness. Meanwhile, cook peppers and onions in remaining dressing mixture in skillet until crisp-tender. With knife slanted, cut steak across the grain into thin slices. Serve topped with the vegetable mixture.