In a large bowl, combine the sweet chili sauce, soy sauce, rice vinegar, minced garlic, minced ginger, vegetable oil, sesame oil, 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.
Remove the chicken from the marinade, allowing any excess to drip off. Discard the remaining marinade.
Grill the chicken for about 6-7 minutes per side, or until the internal temperature reaches 165°F and the chicken is nicely caramelized. If using a skillet, cook for the same time, flipping halfway through.
Once cooked, remove the chicken from the heat and let it rest for 5 minutes before slicing.
Serve the chicken garnished with chopped green onions and sesame seeds.