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 sugars are fully dissolved and the mixture is smooth.
02 - Submerge the chicken thighs in the marinade, turning each piece to ensure even, thorough coating. Cover the bowl and refrigerate for a minimum of 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, allowing excess liquid to drip back into the bowl. Reserve all remaining marinade. Place the thighs skin-side up on the prepared baking sheet, spacing them apart for even air circulation.
05 - Roast the chicken for 35 to 40 minutes. After the first 20 minutes, open the oven and baste the thighs generously with the reserved marinade using a basting brush. Continue roasting until the internal temperature reaches 165°F (75°C) at the thickest part and the skin is deeply 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. For a thicker glaze, whisk the cornstarch into the water until dissolved, then stir the slurry into the boiling sauce. Reduce the heat and simmer for 3 to 5 minutes, stirring occasionally, until the sauce becomes glossy and coats the back of a spoon.
07 - Transfer the roasted chicken thighs to a warmed serving platter. Drizzle generously with the reduced teriyaki sauce. Scatter the sliced green onions and toasted sesame seeds over the top. Serve immediately while hot.