Make the Teriyaki Sauce:
In a medium saucepan, combine soy sauce, honey, rice vinegar, brown sugar, garlic, ginger, and sesame oil. Heat over medium until it starts to simmer.

Thicken the Sauce:
In a small bowl, whisk the cornstarch with cold water until smooth. Slowly pour it into the simmering sauce while whisking constantly. Let it cook for 2–3 minutes, or until thickened. Remove from heat and cool to room temperature.

Marinate the Chicken:
Place chicken thighs in a shallow dish or resealable bag. Pour ½ cup of the cooled sauce over the chicken, turning to coat evenly. Reserve the rest of the sauce for later. Cover and refrigerate for at least 1 hour (up to 8 hours for more flavor).

Grill the Chicken:
Preheat your grill to medium-high heat (around 400–450°F). Clean and lightly oil the grates. Remove chicken from marinade and discard any remaining marinade in the bag.

Cook on the Grill:
Grill the chicken for 6–8 minutes on each side, or until fully cooked (internal temp of 165°F). Baste with some reserved sauce during the last couple minutes of grilling.

Rest and Serve:
Let the grilled chicken rest for 5 minutes off the heat. Then brush with more of the remaining teriyaki sauce and sprinkle with chopped green onions and sesame seeds before serving.