In a large bowl, combine the sweet chili sauce, soy sauce, rice vinegar, vegetable oil, minced garlic, grated ginger, black pepper, and red pepper flakes (if using). Mix well to create the marinade.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover the bowl with plastic wrap and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Preheat your grill or a large skillet over medium-high heat. If using a skillet, add a little oil to prevent sticking.
Remove the chicken from the marinade, allowing excess marinade to drip off. Discard the remaining marinade.
Grill or cook the chicken for about 6-7 minutes per side, or until the internal temperature reaches 165°F and the chicken is nicely browned.
Once cooked, remove the chicken from the heat and let it rest for 5 minutes.
Slice the chicken and serve it garnished with sliced green onions and sesame seeds.