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 each piece to coat evenly. Cover and refrigerate for at least 30 minutes, or up to overnight for deeper flavor penetration.
03 - Set the oven to 400°F. Line a rimmed baking sheet with parchment paper or aluminum foil for easier cleanup.
04 - Remove the chicken thighs from the marinade, holding onto all the remaining liquid. Place the thighs skin-side up on the prepared baking sheet, spacing them apart so they brown 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 and the skin turns a deep golden brown.
06 - While the chicken roasts, pour the remaining marinade into a small saucepan and bring to a rolling boil. For a thicker glaze, dissolve the cornstarch in cold water and whisk it into the boiling sauce. Simmer for 3 to 5 minutes until glossy and coats the back of a spoon.
07 - Transfer the roasted chicken thighs to a warm serving platter. Drizzle with the reduced teriyaki sauce, then scatter the sliced green onions and toasted sesame seeds over the top. Serve immediately while hot.