In a large bowl, combine the sweet chili sauce, soy sauce, rice vinegar, minced garlic, minced ginger, vegetable oil, sesame oil, and red pepper flakes (if using). Whisk until well blended.
Add the chicken thighs to the marinade, ensuring they are fully coated. Cover 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, letting any excess 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 caramelized.
Remove the chicken from the heat and let it rest for 5 minutes before slicing.
Serve sliced chicken garnished with sliced green onions and sesame seeds.