Make the marinade: In a bowl, whisk soy sauce, mirin, honey, garlic, ginger, sesame oil, and red pepper flakes (if using). Taste and adjust sweetness or salt as needed.
Marinate the chicken: Add chicken thighs to a zip-top bag or shallow dish. Pour in half the marinade.
Reserve the other half for the sauce. Marinate for 20–30 minutes at room temperature or up to 8 hours in the fridge.
Prep the sauce base: In a small cup, stir cornstarch into the water until smooth. Set aside.
This slurry will thicken the reserved marinade.
Cook the chicken (stovetop method): Heat the oil in a large skillet over medium-high. Shake excess marinade off the thighs and lay them in the pan. Cook 5–6 minutes per side until browned and cooked through, reaching 165°F/74°C.
Thicken the sauce: While the chicken rests on a plate, pour the reserved marinade into the same pan.
Bring to a gentle simmer. Stir in the cornstarch slurry and cook 1–2 minutes until glossy and thick.
Coat and finish: Return the chicken to the pan and spoon the sauce over to coat. Simmer 1 minute to marry flavors.
Serve: Slice or leave whole.
Top with scallions and sesame seeds. Serve with rice and veggies, spooning extra sauce on top.
Grill: Preheat to medium-high. Grill 5–6 minutes per side.
Warm and thicken the reserved marinade on the stove with the slurry, then brush over the grilled thighs.
Oven: Bake at 425°F (220°C) for 18–22 minutes. Broil 1–2 minutes for caramelized edges. Thicken sauce separately and pour over before serving.
Air fryer: 390°F (200°C) for 14–16 minutes, flipping halfway.
Thicken sauce on the stove and toss to coat.