1. Trim the flank steak and place in a flat pan. Mix the mustard, vinegar, vermouth, rosemary, pepper and oil and pour over steak. Cover and refrigerate overnight, turning several times.
2. Drain meat and pat dry with paper towel. Grill at high heat for 3 minutes each side. Baste meat with marinade. Cook for an additional 3-4 minutes.
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