Preheat your oven to 400°F (200°C).
In a large bowl, combine the sweet chili sauce, soy sauce, honey, minced garlic, ground ginger, vegetable oil, sesame oil, rice vinegar, black pepper, and red pepper flakes (if using). Mix well to create the marinade.
Add the chicken thighs to the bowl, ensuring they are well coated in the marinade. Cover and let them marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.
Place the marinated chicken thighs on a baking sheet lined with parchment paper, skin side up.
Bake in the preheated oven for 35-40 minutes, or until the chicken is cooked through and the skin is crispy. Baste with the remaining marinade halfway through cooking for extra flavor.
Once done, remove from the oven and let the chicken rest for 5 minutes. Garnish with chopped green onions and sesame seeds before serving.