In a large bowl, whisk together soy sauce, brown sugar, honey, rice vinegar, garlic, and ginger until well combined.
Add the chicken thighs to the marinade, ensuring they are fully coated. Cover and refrigerate for at least 30 minutes, or up to 4 hours for more flavor.
Preheat your grill or a large skillet over medium-high heat. Remove the chicken from the marinade, reserving the marinade for later.
Cook the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F.
While the chicken is cooking, pour the reserved marinade into a small saucepan. Bring to a boil over medium heat.
In a small bowl, mix cornstarch and water to create a slurry. Add this to the boiling marinade and stir until thickened, about 1-2 minutes.
Once the chicken is cooked, remove it from the heat and let it rest for a few minutes. Slice the chicken and drizzle with the thickened teriyaki sauce.
Garnish with sesame seeds and sliced green onions before serving.