In a medium bowl, whisk together the soy sauce, brown sugar, honey, rice vinegar, minced garlic, grated ginger, and sesame oil until well combined.
Reserve about 1/4 cup of the marinade in a small bowl for basting later. Pour the remaining marinade over the chicken thighs in a large zip-top bag or shallow dish. Seal or cover and marinate in the refrigerator for at least 30 minutes, or up to 4 hours for enhanced flavor.
Preheat your grill to medium-high heat. Remove the chicken from the marinade and discard the used marinade. Grill the chicken thighs for about 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C) and the chicken has nice grill marks.
While the chicken is grilling, prepare the reserved marinade. In a small saucepan, combine the reserved marinade with cornstarch and water. Bring to a boil over medium heat, stirring constantly, until the sauce thickens slightly.
Once the chicken is cooked, remove it from the grill and let it rest for 5 minutes. Slice the chicken and drizzle the thickened teriyaki sauce over the top.
Garnish with sesame seeds and chopped green onions before serving. Enjoy your grilled teriyaki chicken with rice or steamed vegetables.