Step 1: Pat the chicken thighs dry with paper towels and cut them into 1-inch bite-sized pieces.
Step 2: In a medium bowl, whisk together the soy sauce, mirin, sake (if using), brown sugar, grated fresh ginger, minced garlic, and sesame oil to create the marinade and sauce base.
Step 3: Add the chicken pieces to the marinade, ensuring they are well coated. Cover the bowl and refrigerate for at least 15-20 minutes, or up to 2 hours.
Step 4: Heat a large skillet or wok over medium-high heat. Add 1 tablespoon of neutral oil. Once shimmering, add the marinated chicken pieces in a single layer, cooking in batches if necessary to avoid overcrowding.
Step 5: Cook the chicken for 3-4 minutes per side, until beautifully browned and cooked through. Remove the cooked chicken from the pan and set aside on a plate.
Step 6: Deglaze the pan with a splash of water or sake, scraping up any browned bits from the bottom.
Step 7: Pour the remaining marinade from the bowl into the skillet. Bring to a simmer over medium heat.
Step 8: In a small bowl, whisk together the cornstarch and 2 tablespoons of cold water to create a smooth slurry.
Step 9: Slowly whisk the cornstarch slurry into the simmering sauce in the skillet. Continue to whisk gently as the sauce thickens into a glaze, usually within 1-2 minutes.
Step 10: Return the cooked chicken to the skillet with the thickened teriyaki glaze. Toss gently to coat all the chicken pieces thoroughly.
Step 11: Cook for another 1-2 minutes, allowing the chicken to warm through and soak up the sauce.
Step 12: Garnish with toasted sesame seeds and sliced green onions just before serving. Serve hot over rice or noodles.