In a medium bowl, whisk together brown sugar, minced garlic, soy sauce, apple cider vinegar, sesame oil, grated ginger, black pepper, and red pepper flakes until the sugar is dissolved and the mixture is well combined.
Place the chicken thighs in a large resealable plastic bag or a shallow dish. Pour the marinade over the chicken, ensuring they are fully coated. Seal the bag or cover the dish and marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.
Preheat your oven to 400°F (200°C).
Arrange the marinated chicken thighs on a baking sheet lined with parchment paper, skin side up. Reserve the marinade for later.
Bake the chicken in the oven for 30 minutes, then remove it and brush the reserved marinade over the chicken.
Return the chicken to the oven and bake for an additional 10-15 minutes, or until the skin is crispy and the internal temperature reaches 165°F (75°C).
Once cooked, remove the chicken from the oven and let it rest for 5 minutes before serving.
Garnish with freshly chopped green onions before serving to add a touch of color and freshness.
Notes
Serve with steamed rice or mixed vegetables for a complete meal.