01 - Whisk together miso paste, soy sauce, mirin, honey, sesame oil, garlic, ginger, rice vinegar, and black pepper in a large bowl until smooth and well combined.
02 - Add chicken thighs to the marinade, turning to coat thoroughly on all sides. Cover and refrigerate for at least 30 minutes, up to 4 hours for deeper flavor penetration.
03 - Preheat oven to 400°F. Line a baking tray with parchment paper or aluminum foil for easy cleanup.
04 - Remove chicken from marinade, shaking off excess liquid. Arrange thighs in a single layer on the prepared baking tray, leaving space between pieces for even cooking.
05 - Roast for 25 to 30 minutes until chicken is cooked through and edges are caramelized and golden brown. Internal temperature should reach 165°F.
06 - Transfer chicken to a serving platter. Sprinkle with toasted sesame seeds and finely sliced spring onions if desired. Serve immediately while hot.