Prep the chicken. Pat the chicken dry and cut into bite-size pieces or keep thighs whole.
Season lightly with salt only if using low-sodium soy sauce.
Make the teriyaki sauce. In a small bowl, whisk soy sauce, mirin, sake (or water), brown sugar, garlic, and ginger. Taste and adjust sweetness or saltiness. If you want a thicker glaze, whisk in the cornstarch.
Heat the pan. Preheat a large skillet over medium-high heat.
Add oil and swirl to coat.
Sear the chicken. Add the chicken in a single layer. Cook without moving for 3–4 minutes to get browning, then flip and cook another 3–4 minutes until nearly cooked through.
Add the sauce. Lower the heat to medium. Pour in the sauce and stir.
It will simmer and reduce. As it thickens, toss the chicken to coat. This takes 2–4 minutes.
Finish and glaze. When the sauce looks glossy and clings to the chicken, turn off the heat.
Stir in a small drizzle of sesame oil if you like.
Garnish and serve. Sprinkle with green onions and sesame seeds. Serve over hot rice with steamed or stir-fried veggies.