In a large zip-top bag, combine all ingredients. Squeeze out air and seal bag. Massage bag until well combined and marinade mix is dissolved. Marinate in refrigerator for a minimum of 1 hour or up to 8 hours. Remove flank steak from refrigerator 30 minutes before cooking.
Preheat grill to medium-high heat. Oil grate when ready to start cooking.
Remove steak from marinade; discard marinade. Place steak on grate. Grill for 5 to 6 minutes per side for medium.
Remove from grill and let rest for 5 minutes.
To serve, slice across the grain.