In a bowl, whisk together soy sauce, sugar, vinegar, garlic, ginger, sesame oil, and chili flakes.
Marinate the steak for at least 30 minutes (1-2 hours preferred).
Preheat grill to high
When ready to cook, brush and oil the grill grate. Place the flank steak on the hot grill until cooked to taste, 4 to 6 minutes per side for medium-rare (about 120°F on an instant-read meat thermometer). Transfer to a cutting board and let rest for 5 minutes. Cut the steak into paper-thin slices by holding the knife blade at a 45 degree angle to the top of the meat.