01 - In a large mixing bowl, whisk together the soy sauce, mirin, honey, brown sugar, rice vinegar, minced garlic, grated ginger, and sesame oil until fully combined.
02 - Add the chicken thighs to the bowl and turn to coat evenly. Cover and refrigerate for at least 30 minutes, or up to overnight for deeper flavor penetration.
03 - Preheat the oven to 400°F (200°C). Line a baking tray with parchment paper or aluminum foil for easier cleanup.
04 - Remove the chicken thighs from the marinade, reserving all remaining liquid. Place the thighs skin-side up on the prepared baking tray, spacing them evenly.
05 - Roast the chicken for 35 to 40 minutes, basting with the reserved marinade after the first 20 minutes. Continue roasting until the internal temperature reaches 165°F (75°C) and the skin is golden brown.
06 - While the chicken roasts, pour the remaining marinade into a small saucepan and bring to a boil. For a thicker glaze, dissolve the cornstarch in water and stir it in. Simmer for 3 to 5 minutes until the sauce is glossy and coats the back of a spoon.
07 - Transfer the roasted chicken to a serving platter. Drizzle generously with the reduced teriyaki sauce, then garnish with sliced green onions and toasted sesame seeds. Serve immediately while hot.