Combine all the ingredients in a bowl and whisk. Add the flank steak and coat with the marinade. Set aside for 5 minutes.
Heat a grill pan to high heat. Sear the flank steak on one side for 3-4 minutes, then rotate so the grill marks are diagonal and cook for another minute.
Flip, cook for another 34 minutes, turn off the heat and remove from the pan.
Allow to rest fro several minutes before cutting on the diagonal, against the grain.