Preheat the oven to 400°F (200°C).
In a large bowl, drizzle 1 tablespoon sesame oil over the chicken thighs and toss to coat.
Add 3 tablespoons cornstarch, 1 teaspoon garlic powder, 1 teaspoon salt, and 1/2 teaspoon black pepper to the bowl and stir until each thigh is evenly coated.
Arrange the coated chicken thighs in a single layer in a 9×13-inch baking dish.
Bake the chicken thighs for 12 minutes in the preheated oven.
While the chicken bakes, make the teriyaki glaze: combine 1/2 cup soy sauce, 1/4 cup water, 1/4 cup brown sugar, 1/4 cup honey, 1 tablespoon rice vinegar, 2 teaspoons minced garlic, 1 teaspoon minced fresh ginger, and 1 teaspoon sesame oil in a small saucepan and bring to a boil over medium-high heat.
Reduce heat and simmer the sauce for 1 minute, then remove from heat. Stir the cornstarch slurry (1 tablespoon cornstarch mixed with 2 teaspoons water) into the hot sauce to thicken, stirring until smooth.
Remove the chicken from the oven and pour the teriyaki glaze evenly over the thighs. Use tongs to turn each thigh so it is coated on all sides.
Return the glazed chicken to the oven and bake an additional 6–10 minutes, or until an instant-read thermometer inserted into the thickest part reaches 165°F (74°C).
Remove the chicken from the oven and turn the thighs once more to coat in the glaze. The sauce will thicken as it cools. Garnish with finely chopped green onions if desired and serve.