Tangy balsamic vinegar makes this marinade. Mix in Dijon mustard, garlic and basil, pour over Flank Steak, then grill and serve with a variety of veggies for a simple, fresh meal.
Combine marinade ingredients in small bowl. Place beef Flank Steak and marinade in food-safe plastic bag; turn steak to coat. Close bag securely and marinate in refrigerator 6 hours or as long as overnight, turning occasionally.
Remove steak from marinade; discard marinade. Place steak on grid over medium, ash-covered coals. Grill, covered, 11 to 16 minutes (over medium heat on preheated gas grill, 16 to 21 minutes) for medium rare (145°F) to medium (160°F) doneness, turning occasionally.
Season steak with salt and pepper, as desired. Carve steak crosswise into thin slices.