In a small saucepan, combine the soy sauce, mirin, brown sugar, sesame oil, garlic, and ginger. Heat the mixture over medium heat, stirring occasionally, until it begins to simmer.

Stir in the cornstarch slurry and cook for 1-2 minutes, or until the sauce thickens and becomes glossy. Remove from heat and let it cool completely.

Place the chicken thighs in a large bowl or resealable plastic bag. Pour half of the teriyaki sauce over the chicken, reserving the other half for basting and serving. Toss the chicken to coat evenly, then cover and marinate in the refrigerator for 20-30 minutes.

While the chicken marinates, preheat your oven to 400°F (200°C). Line a baking sheet with aluminum foil and lightly grease it with cooking spray.

Arrange the marinated chicken thighs on the prepared baking sheet, shaking off any excess marinade. Bake for 25-30 minutes, flipping the chicken halfway through, until the internal temperature reaches 165°F (74°C).

During the last 5 minutes of baking, brush the chicken with some of the reserved teriyaki sauce. Switch the oven to broil and cook for 2-3 minutes, or until the sauce caramelizes and the chicken develops a sticky, glossy finish. Watch closely to prevent burning.

Remove the chicken from the oven and let it rest for 5 minutes. Garnish with sliced green onions and sesame seeds before serving. Enjoy!