In a medium bowl, whisk together soy sauce, mirin, brown sugar, rice vinegar, minced garlic, and grated ginger until well combined. Set aside for 5 minutes.

Place the chicken thighs in a resealable plastic bag or shallow dish, pour the marinade over, and refrigerate for at least 30 minutes.

Heat a large skillet over medium-high heat. Remove chicken from marinade, sear each side for about 6-7 minutes or until golden brown and cooked to 165°F.

Pour remaining marinade into a saucepan and bring to a simmer. Mix cornstarch with water to create a slurry and add to the marinade, stirring until thickened.

Let the chicken rest, slice, and drizzle thickened sauce over the chicken before serving.