Add all ingredients except the steak to a mixing bowl. Whisk together until fully combined.
Pierce the flank steak with a fork all over, and then place in the bowl*.
Turn the steak several times to coat thoroughly.
Cover the bowl with plastic wrap and place in the fridge for 2-24 hours.
Remove from the fridge 30 minutes before grilling. Take the steak out of the marinade and remove excess moisture with paper towels. Discard the marinade**.
Preheat grill for medium-high heat, about 400°F (204°C).
Spray the grill grates with cooking oil spray.
When hot, place flank steak on the grill.
Grill the first side for 1 minute to sear. Then flip the steak and grill 1 minute on the other side.
Continue cooking for 2-3 minutes per PER SIDE to desired doneness***, flipping every minute for even cooking.
Remove to a cutting board and let the steak rest covered with aluminum foil for 5 minutes (this is so the juices retreat into the meat).
Slice thinly against the grain to serve.