Preheat your oven to 400°F (200°C).
In a large bowl, whisk together the sweet chili sauce, soy sauce, honey, rice vinegar, minced garlic, grated ginger, black pepper, and red pepper flakes if using.
Add the chicken thighs to the bowl and toss until they are well coated with the marinade. Let them marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more flavor.
Place the marinated chicken thighs skin-side up on a baking sheet lined with parchment paper.
Bake in the preheated oven for 35-40 minutes, or until the chicken is cooked through and the skin is crispy. Baste the chicken with the 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.