In a medium bowl, whisk together soy sauce, brown sugar, rice vinegar, sesame oil, garlic, ginger, black pepper, and red pepper flakes.
Place the sliced flank steak in a resealable plastic bag or shallow dish and pour the marinade over the beef. Seal the bag or cover the dish and refrigerate for at least 1 hour, preferably overnight for maximum flavor.
Preheat a grill or grill pan over medium-high heat. Remove the beef from the marinade, allowing excess marinade to drip off.
Grill the beef slices for about 2-3 minutes on each side, or until cooked to your desired doneness. Avoid overcrowding the grill; cook in batches if necessary.
Once cooked, remove the beef from the grill and let it rest for a few minutes. Slice into bite-sized pieces if desired.
Garnish with chopped green onions and sesame seeds before serving.