01 -  In a medium bowl, whisk together the soy sauce, ginger, garlic, honey, rice vinegar, sesame oil, black pepper, and red pepper flakes (if using). 
 02 -  Place the chicken breasts in a large resealable bag or shallow dish. Pour the marinade over the chicken, ensuring all pieces are well coated. Seal and refrigerate for at least 1 hour, or up to 8 hours for deeper flavor. 
 03 -  Preheat grill to medium-high heat. Lightly oil the grates. 
 04 -  Remove chicken from marinade, letting excess drip off. Discard the used marinade. Grill chicken for 6–8 minutes per side, or until cooked through and an internal temperature of 165°F (74°C) is reached. 
 05 -  Transfer chicken to a plate and let rest for 5 minutes. Slice and garnish with green onions and sesame seeds before serving.