Preheat the oven to 425°F and line a baking sheet with parchment paper or foil. This ensures even cooking and makes cleanup much easier afterward.
In a bowl, mix miso paste, brown sugar, rice vinegar, butter, ginger, pepper flakes, and black pepper until smooth. The mixture should be thick yet spreadable for easy coating.
Pat chicken thighs dry and trim excess fat if needed. Rub the glaze evenly over each piece so every bite gets maximum flavor.
Arrange chicken on the baking sheet and roast for about 25 minutes. Flip halfway through and spoon any glaze from the pan back over the chicken for even caramelization.
Once cooked through and nicely browned, remove from oven and rest briefly. Garnish with sliced green onions before serving for freshness and color.