DO-AHEAD TIP: Marinate flank steak overnight (see recipe).
In a large zipper-topped plastic bag, combine vinegar, oil, sugar, soy
sauce, half the green onions, the garlic, and pepper; add flank steak, seal bag and turn to coat;
refrigerate overnight.
Preheat outdoor or indoor grill to MEDIUM-HIGH; remove steak from bag (discard marinade and bag)
and grill to desired level of doneness.
Transfer steak to a cutting board and allow it to rest for 10 minutes before thinly slicing across the
grain (diagonally); garnish with remaining chopped green onion.