Pat the flank steak dry on both sides with paper towels and lightly score the top in a shallow diagonal crosshatch about 1½ inches apart.
Place the steak in a zip-top bag and pour in the marinade; remove as much air as possible, seal, and turn the bag to coat the steak evenly.
Place the bag on a plate or in a shallow dish to catch drips and refrigerate for at least 4 hours or up to 24 hours.
When ready to grill, remove the steak from the refrigerator and let it sit at room temperature for 20 minutes.
Heat a gas or charcoal grill to medium-high (about 425–450°F) and clean the grates.
Remove the steak from the marinade and shake off excess; discard the marinade. Grill the steak about 3–4 minutes per side until an instant-read thermometer reads 130°F for medium-rare (cook 1–2 minutes more per side for more well done).
Transfer the steak to a plate, cover loosely, and let rest for 10 minutes.
Slice the rested steak very thinly (about 1/8 inch) across the grain and serve warm.