Combine the first 5 ingredients in a small bowl to make the Seasoning Mix and stir until combined.
Season the flank steak with 1 tablespoon of the Seasoning Mix per side, 2 tablespoons total. Wrap the steak in plastic wrap and place in the refrigerator overnight.
Prepare a charcoal or gas grill and preheat until the coals are very hot. The grilling surface should be about 5 inches from the coals.
Cook the steak on the grill, turning once, until the meat is seared on the outside but still rare inside, about 6 minutes per side. To serve, slice thinly across the grain. Note: charcoal grills vary widely in heat output, so you may have to adjust the cooking times accordingly.