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 smooth.
02 - Add the chicken thighs to the bowl and turn each piece to coat thoroughly in the marinade. Cover and refrigerate for at least 30 minutes, or up to overnight for deeper flavor penetration.
03 - Preheat the oven to 400°F. Line a rimmed baking tray with parchment paper or aluminum foil for easier cleanup.
04 - Remove the chicken thighs from the marinade, allowing excess to drip back into the bowl. Reserve all remaining marinade 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 generously with the reserved marinade after the first 20 minutes. Continue roasting until the internal temperature of the thickest piece registers 165°F 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, glossier glaze, whisk the cornstarch into cold water until smooth, then stir the slurry into the saucepan. Simmer for 3 to 5 minutes, stirring frequently, until the sauce coats the back of a spoon.
07 - Transfer the roasted chicken thighs to a warm serving platter. Drizzle generously with the reduced teriyaki glaze. Scatter the sliced green onions and toasted sesame seeds over the top. Serve immediately while hot.