In a medium bowl, whisk together the soy sauce, mirin, brown sugar, 1/4 cup (60 ml) water, rice vinegar, sesame oil, garlic, and ginger until the sugar is mostly dissolved.

In a small bowl, whisk the cornstarch with 2 tablespoons (30 ml) cold water until completely smooth. Set aside.

Pat the chicken thighs dry with paper towels and cut them into evenly sized 1 1/2-inch (4 cm) pieces. Season with the black pepper.

Heat the neutral oil in a 12-inch skillet over medium-high heat until shimmering.

Add the chicken in a single layer: If necessary, cook it in two batches rather than overcrowding the skillet. Let it cook undisturbed for 3 to 4 minutes, until the underside develops a deep golden-brown color.

Turn the chicken pieces and cook for another 3 to 5 minutes. They should be browned on several sides and nearly cooked through.

Reduce the heat to medium: Pour the prepared teriyaki sauce into the skillet and stir to coat the chicken. Bring it to a gentle simmer.

Simmer for 2 to 3 minutes, stirring occasionally, until the thickest chicken pieces reach at least 165°F (74°C).

Stir the cornstarch slurry again, then slowly pour it into the simmering skillet while stirring. Cook for 1 to 2 minutes. The sauce should become glossy and thick enough to cling to the chicken.

If the sauce becomes too thick, stir in 1 tablespoon (15 ml) water at a time until it loosens. If it remains too thin, simmer for another minute.

Toss the chicken through the thickened sauce until every piece has an even glaze and the skillet no longer has a pool of thin liquid at the bottom.

Remove from the heat: Sprinkle with sliced green onions and toasted sesame seeds, if using, and serve hot.