In a large bowl, combine sweet chili sauce, soy sauce, rice vinegar, minced garlic, minced ginger, vegetable oil, sesame oil, and red pepper flakes (if using). Whisk until well combined.
Add the chicken thighs to the bowl, ensuring they are well coated in the marinade. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Preheat your grill or a 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. Grill or cook the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F and the chicken is nicely caramelized.
Once cooked, remove the chicken from the heat and let it rest for 5 minutes. Slice the chicken and serve it drizzled with any remaining sauce from the cooking process.
Garnish with sliced green onions and sesame seeds before serving.