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Combine the balsamic vinegar, Dijon mustard, olive oil, onion powder and garlic powder in a large resealable zip-top bag. Add the flank steak and seal the bag. Refrigerate for 30 minutes to 2 hours, turning the bag over several times during the marinating time.
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Remove the steak from the marinade and discard the remaining marinade. Season the steak with salt and pepper.
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Heat a grill on high for 10-15 minutes.
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Grill the steak to your desired doneness, about 4 minutes per side for medium rare (depending on your grill).