In a medium saucepan, whisk together the soy sauce, mirin, honey, rice vinegar, brown sugar, minced garlic, grated ginger, and sesame oil. Bring the mixture to a simmer over medium heat.

In a small bowl, whisk together the cornstarch and cold water until smooth. Pour the cornstarch slurry into the simmering sauce, whisking constantly. Continue to cook, stirring frequently, until the sauce thickens and coats the back of a spoon, about 2-3 minutes.

Remove the sauce from the heat and let it cool to room temperature.

Place the chicken thighs in a large, shallow dish or resealable plastic bag. Pour 1/2 cup of the cooled teriyaki sauce over the chicken, reserving the remaining sauce for basting and serving. Toss the chicken to coat evenly with the sauce. Cover the dish or seal the bag and refrigerate for at least 1 hour, or up to 8 hours, to marinate.

Preheat the grill to medium-high heat (400-450°F). Clean and oil the grill grates.

Remove the chicken thighs from the marinade, discarding the used marinade. Grill the chicken thighs for 6-8 minutes per side, or until the internal temperature reaches 165°F. Baste the chicken with some of the reserved teriyaki sauce during the last 2-3 minutes of grilling.

Remove the grilled teriyaki chicken from the grill and let it rest for 5 minutes.

Brush the chicken with the remaining teriyaki sauce and garnish with chopped green onions and toasted sesame seeds.