Preheat your oven to 375°F (190°C).
In a mixing bowl, combine the brown sugar, soy sauce, minced garlic, olive oil, ground ginger, black pepper, and optional red pepper flakes. Stir until the mixture is smooth and the sugar has dissolved.
Arrange the chicken thighs in a baking dish and pour the sauce over them, making sure each piece is thoroughly coated.
Bake for approximately 25-30 minutes, or until the chicken is tender and reaches an internal temperature of 165°F (74°C).
Once done, take the dish out of the oven and allow the chicken to rest for about 5 minutes.
Finish by sprinkling the chopped green onions on top before serving.