- In a large bowl, whisk together the soy sauce, ketchup, brown sugar, pineapple juice, rice vinegar, sesame oil, garlic, ginger, black pepper, and red pepper flakes until well combined. 
- Add the chicken thighs to the marinade, ensuring they are fully coated. Cover and refrigerate for at least 2 hours, or overnight for best flavor. 
- Preheat your grill to medium-high heat. Remove the chicken from the marinade, reserving the marinade for basting. 
- Place the chicken skin-side down on the grill and cook for about 6-8 minutes, or until the skin is crispy and golden brown. 
- Flip the chicken and baste with the reserved marinade. Grill for an additional 6-8 minutes, or until the internal temperature reaches 165°F. 
- Remove the chicken from the grill and let it rest for 5 minutes. Garnish with sliced green onions and sesame seeds before serving.