Prep the chicken: Pat chicken dry.
Trim excess fat. Season lightly with salt and pepper on both sides.
Whisk the sauce: In a bowl, combine soy sauce, honey, mirin, rice vinegar, sesame oil, garlic, ginger, and red pepper flakes. Stir until the honey dissolves.
Set aside.
Sear the chicken: Heat oil in a large skillet over medium-high. When hot, add chicken in a single layer. Sear 4–5 minutes per side until browned.
Work in batches if needed to avoid steaming.
Add sauce and simmer: Reduce heat to medium. Pour the sauce around the chicken. Simmer 3–4 minutes, turning the chicken once, until it starts to thicken slightly.
Glaze it: Stir the cornstarch slurry, then drizzle it into the pan while stirring the sauce.
Simmer 1–2 more minutes until glossy and thick enough to coat a spoon.
Finish and rest: Stir in the white parts of the green onions. Remove from heat and rest 3 minutes. This helps the juices settle and the glaze cling.
Slice and serve: Slice chicken against the grain or serve whole.
Spoon plenty of sauce over the top. Garnish with green onion tops and sesame seeds. Serve with rice and veggies.