coarse kosher salt and freshly ground black pepper
Instructions
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.
Remove the steak from the marinade and discard the remaining marinade. Season the steak with salt and pepper.
Heat a grill on high for 10-15 minutes.
Grill the steak to your desired doneness, about 4 minutes per side for medium rare (depending on your grill).
Let the steak rest for 5-10 minutes, then against the grain and serve immediately.