1. Place steak in large resealable plastic food storage bag. Combine 1/2 cup vinegar, oil, 1/4 cup mustard, garlic, 1/2 teaspoon salt and pepper in small bowl. Pour over steak; turn steak to coat evenly. Seal bag and marinate in refrigerator 1 hour.
2. To prepare Onion Marmalade, melt butter in large skillet over medium-high heat. Add onions; cook until very tender, stirring constantly. Add 1/3 cup mustard, 1/4 cup vinegar and 3/4 teaspoon salt. Cook over medium-low heat until mixture thickens, stirring often.
3. Place steak on grid, reserving marinade. Grill over high heat 15 minutes for medium rare or to desired doneness, turning and basting often with marinade. Do not baste during last 5 minutes of cooking. Slice steak diagonally into thin pieces and serve with Onion Marmalade.
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