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 the sugar dissolves and the mixture is well blended.
02 - Submerge the chicken thighs in the marinade, turning each piece to coat evenly. Cover the bowl and refrigerate for at least 30 minutes, or up to overnight for deeper flavor penetration.
03 - Set the oven to 400°F (200°C). Line a rimmed baking sheet with parchment paper or aluminum foil to simplify cleanup.
04 - Remove the chicken thighs from the marinade, letting excess drip off. Reserve all remaining marinade liquid. Place the thighs skin-side up on the prepared baking sheet, spacing them evenly.
05 - Roast the chicken for 35 to 40 minutes. After the first 20 minutes, baste generously with the reserved marinade. Continue roasting until the internal temperature reaches 165°F (75°C) and the skin is golden and caramelized.
06 - While the chicken roasts, pour the remaining marinade into a small saucepan and bring to a rolling boil over medium-high heat. If a thicker glaze is desired, dissolve the cornstarch in cold water and whisk it into the boiling sauce. Simmer for 3 to 5 minutes, stirring occasionally, until the sauce is glossy and coats the back of a spoon.
07 - Transfer the roasted chicken thighs to a warm serving platter. Drizzle generously with the reduced teriyaki sauce. Finish with sliced green onions and a sprinkle of toasted sesame seeds. Serve immediately while hot.