Place steak in a large resealable plastic bag, pour marinade over, and toss to coat
Marinate, tossing occasionally, at least 30 minutes at room temperature, or chill overnight
If chilled, let steak sit at room temperature 30 minutes
Prepare a grill or grill pan for medium-high heat
Remove steak from marinade; discard marinade
Grill steak until an instant-read thermometer inserted into the center of steak registers 120–125°F for medium-rare, 2–3 minutes per side for skirt; 3–4 minutes per side for flank
Let rest 10 minutes before thinly slicing against the grain (to make slicing easier for skirt steak, cut into 5–6" segments, then slice against the grain)
Transfer to a platter and top with remaining 1/2 tsp