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 - Submerge the chicken thighs in the marinade, turning each piece to coat evenly on all sides. 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. Line a rimmed baking sheet with parchment paper or aluminum foil to catch drips and simplify cleanup.
04 - Remove the chicken 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 at least 1 inch apart for even browning.
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 at the thickest part of the thigh reads 165°F on an instant-read thermometer and the skin is deeply golden and slightly 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 frequently, until the sauce is glossy and coats the back of a spoon.
07 - Transfer the finished chicken to a warm serving platter. Drizzle generously with the reduced teriyaki glaze. Scatter sliced green onions and toasted sesame seeds over the top. Serve immediately while hot.