Begin by preparing the marinade by combining soy sauce, balsamic vinegar, olive oil, Worcestershire sauce, garlic, lemon juice, and Dijon mustard in a bowl.
Place the flank steak in a large ziplock bag and pour the marinade over it, making sure to cover the meat completely.
Seal the bag, removing as much air as possible, and let the steak marinate in the refrigerator for at least 2 hours or overnight for maximum flavor.
Preheat your grill or broiler to high heat, and let the steak come to room temperature before cooking.
Cook the steak for about 5-7 minutes on each side, or until the desired level of doneness is reached.