In a large bowl, combine sweet chili sauce, soy sauce, rice vinegar, minced garlic, grated ginger, vegetable oil, sesame oil, and black pepper. 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 skillet over medium-high heat.
Remove the chicken from the marinade, allowing excess marinade to drip off. Reserve the marinade for later use.
Grill or cook the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F.
While the chicken is cooking, pour the reserved marinade into a small saucepan and bring to a boil. Reduce heat and simmer for 5 minutes to thicken.
Once the chicken is cooked, brush it with the thickened marinade and let it rest for a few minutes.
Slice the chicken and serve it garnished with sliced green onions and sesame seeds.