In a bowl, whisk together the soy sauce, sesame oil, honey, ginger, garlic, rice vinegar, and red pepper flakes (if using).
Add the cubed flank steak to the marinade, ensuring all pieces are well coated. Cover and refrigerate for at least 30 minutes, or up to 4 hours for more flavor.
Preheat your grill or grill pan to medium-high heat. If using wooden skewers, soak them in water for 30 minutes to prevent burning.
Thread the marinated steak cubes onto the skewers, leaving a little space between each piece for even cooking.
Grill the skewers for about 8-10 minutes, turning occasionally, until the steak reaches your desired level of doneness.
During the last minute of grilling, sprinkle the sesame seeds over the skewers and allow them to toast slightly.
Remove the skewers from the grill and let them rest for a few minutes. Garnish with chopped green onions before serving.
