1. Prepare Marinade: In a mixing bowl, combine soy sauce, sesame oil, brown sugar, rice vinegar, ginger, garlic, and red pepper flakes. Mix well until the sugar dissolves.

2. Marinate Chicken: Place chicken thighs in a resealable plastic bag or a bowl. Pour the marinade over the chicken, ensuring it’s well coated. Seal the bag or cover the bowl, and refrigerate for at least 30 minutes.

3. Preheat Grill: Preheat your grill to medium-high heat. Make sure it’s clean and lightly oiled to prevent sticking.

4. Remove Chicken from Marinade: Take the chicken out of the marinade and let any excess drip off. Discard the marinade for safety.

5. Grill Chicken: Place the chicken thighs on the preheated grill. Grill for 6-8 minutes per side or until the internal temperature reaches 165°F (75°C) and the chicken has nice grill marks.

6. Rest Chicken: Once cooked, remove the chicken from the grill and let it rest for a few minutes. This allows the juices to redistribute, keeping the meat moist.

7. Garnish and Serve: Sprinkle chopped green onions and sesame seeds on top of the chicken for added flavor and garnish.