Prep the chicken. Pat thighs dry with paper towels.
Trim excess fat. Season lightly with black pepper (skip extra salt for now since the sauce is salty).
Make the sauce. In a small bowl, whisk soy sauce, mirin, sake (if using), brown sugar, honey, garlic, ginger, and sesame oil. In another small cup, stir the cornstarch and water until smooth.
Set both aside.
Sear the thighs. Heat oil in a large skillet over medium-high. Add chicken in a single layer. Cook 4–5 minutes until browned, then flip and cook another 4–5 minutes.
You want a deep golden crust.
Add the sauce base. Pour the soy mixture into the pan. It will bubble and start reducing. Toss to coat the chicken.
Lower heat to medium.
Thicken the glaze. Stir the cornstarch slurry, then pour it in while mixing the sauce. Simmer 1–2 minutes until the sauce turns glossy and clings to the chicken. If it gets too thick, splash in a tablespoon of water.
Finish with onions. Add the white parts of the green onions and cook 30 seconds.
Turn off the heat. Let the chicken rest in the sauce for 2 minutes to soak up flavor.
Slice and serve. Transfer chicken to a board, slice into strips, and return to the pan to coat. Top with green onion greens and sesame seeds.
Serve over rice with veggies.