Slice the flank steak across and against the grain into ¼ to ½-inch thick slices and place the slices into a large ziplock bag.
Add green onions, soy sauce, vegetable oil, garlic, honey, and vinegar to a blender or food processor and blend until well combined. Pour marinade over steak. Press any extra air from the bag and seal. Refrigerate for 24 hours.
Remove steak strips from the bag and discard leftover marinade. Roll each flank steak strip into a pinwheel shape and secure with a toothpick.
Heat a grill or grill pan to medium-high and lightly grease the grates with oil or cooking spray. Place pinwheels on the grill and cook, undisturbed, until browned and release easily from the grill, about 2 minutes. Flip and cook another 2 to 3 minutes, or until desired doneness, flipping once more.
Transfer cooked pinwheels to a serving platter and keep warm. Serve drizzled with any juices that have accumulated on the platter and sprinkle with sliced green onions if desired.
