Preheat your oven to 375°F (190°C).
In a medium bowl, whisk together the brown sugar, soy sauce, ketchup, minced garlic, ground ginger, olive oil, black pepper, and red pepper flakes until well combined.
Place the chicken thighs in a large baking dish and pour the sauce over them, ensuring they are well coated.
Bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).
For a caramelized finish, broil the chicken for an additional 2-3 minutes after baking.
Remove from the oven and let it rest for 5 minutes before serving. Garnish with sliced green onions.