In a large bowl, whisk together soy sauce, brown sugar, and water until the sugar is dissolved. Stir in rice vinegar, minced garlic, grated ginger, black pepper, and red pepper flakes if desired.

Add chicken thighs to the marinade, coat thoroughly, cover with plastic wrap, and refrigerate for at least 1 hour, preferably overnight.

Preheat your oven to 375°F (190°C) and prepare a baking dish by greasing it lightly.

Transfer the marinated chicken thighs to the prepared baking dish, pouring any remaining marinade over it. Bake for 35-40 minutes until the chicken reaches 165°F (74°C).

Broil for an additional 5 minutes for caramelization, watching closely to prevent burning.

Let the chicken rest for 5 minutes, then garnish with sliced green onions and sesame seeds before serving.