In a medium bowl, whisk together soy sauce, mirin, brown sugar, rice vinegar, minced garlic, and grated ginger. Ensure everything is well combined, and the sugar is dissolved.

Place the chicken thighs in a resealable bag or shallow dish, then pour the freshly made marinade over them. Seal the bag or cover the dish tightly and refrigerate for at least 30 minutes or ideally overnight.

Heat a large skillet or grill pan over medium-high heat for about 3-4 minutes. Remove the chicken from the marinade and add sesame oil to the skillet if desired.

Add the marinated chicken thighs to the hot skillet, searing them for 6-7 minutes on each side until golden brown and cooked through (internal temperature of 165°F/75°C).

Pour any leftover marinade into a small saucepan and bring to a gentle simmer. Prepare a slurry by mixing cornstarch with water and add it to the simmering marinade. Stir until the sauce thickens.

Remove the chicken from the skillet and let it rest for a few minutes. Slice into bite-sized pieces, and drizzle the thickened teriyaki sauce over the top or coat the slices evenly.

Serve your Teriyaki Chicken over rice or noodles, garnished with chopped green onions and sesame seeds.