01 - In a large mixing bowl, whisk together the white miso paste, soy sauce, mirin, honey, sesame oil, minced garlic, grated ginger, rice vinegar, and black pepper. Whisk vigorously until the mixture forms a smooth, homogeneous paste.
02 - Add the boneless chicken thighs to the bowl with the prepared marinade. Turn each piece several times to ensure thorough coating. Cover the bowl tightly with plastic wrap and refrigerate for a minimum of 30 minutes. For optimal flavor development, marinate for up to 4 hours or overnight.
03 - Preheat your oven to 400°F. Line a rimmed baking sheet with parchment paper or aluminum foil for easy cleanup. This preparation step should be completed just before removing the chicken from the refrigerator.
04 - Remove the marinated chicken from the refrigerator. Lift each thigh from the bowl and allow excess marinade to drip off briefly. Place the chicken pieces on the prepared baking sheet, arranging them in a single layer with space between pieces for proper heat circulation.
05 - Place the baking sheet in the preheated oven. Roast for 25 to 30 minutes, or until the chicken is completely cooked through and the edges are nicely caramelized. The internal temperature should reach 165°F when measured with a meat thermometer.
06 - Transfer the roasted chicken to a warmed serving platter. Sprinkle toasted sesame seeds and finely sliced spring onions evenly over the top. Serve immediately while hot, accompanied by steamed rice and sautéed vegetables for a complete meal.