I usually just do a light rub of cumin, pepper, and salt and then cook it quickly on a very hot skillet
Let it get to room temp then trim any external fat. Kosher or sea salt and cracked black pepper, sear each side on a really hot surface for 30 seconds to a minute and a half (depending on surface temp) and let it rest. Make a chimichurri sauce. Slice it against the grain 3/8 inch or so wide and enjoy your rare flank steak with your sauce.