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 combined.
02 - Add the chicken thighs to the marinade, turning each piece to ensure thorough, even coating. 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 rimmed baking sheet with parchment paper or aluminum foil for easier cleanup.
04 - Remove the chicken thighs from the marinade, gently shaking off excess. Reserve all remaining marinade liquid. Place the thighs skin-side up on the prepared baking sheet, spacing them at least 1 inch apart.
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 part reaches 165°F (74°C) 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. If a thicker glaze is desired, dissolve the cornstarch in cold water and whisk it into the boiling sauce. Reduce the heat and 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 thickened teriyaki sauce. Scatter the sliced green onions and toasted sesame seeds over the top. Serve immediately while hot.