Pat the chicken thighs dry with paper towels to remove excess moisture.

Season both sides of each chicken thigh with salt and black pepper.

In a small bowl, whisk together 1/2 cup soy sauce, 1/4 cup mirin, 3 tablespoons brown sugar, 4 cloves minced garlic, 1 tablespoon minced ginger, 1 tablespoon rice vinegar, 1/4 teaspoon red pepper flakes, and 1 teaspoon sesame oil until the sugar dissolves and the glaze is smooth.

Heat a 12-inch skillet over medium-high heat and add 2 tablespoons vegetable oil once the pan is hot.

Place the chicken thighs skin-side down in the hot skillet and cook without moving for 6 to 7 minutes until the skin is golden brown.

Flip the chicken thighs and cook the other side for 3 to 4 minutes until lightly browned.

Pour the prepared shoyu glaze over the chicken, reduce heat to medium, and simmer for 5 to 8 minutes, turning the chicken occasionally to coat evenly.

Check that the chicken reaches 165°F in the thickest part or that the juices run clear; the glaze should be thick and glossy.

Transfer the chicken to a serving platter, spoon any remaining glaze over the top, and garnish with sliced green onions and sesame seeds.