Prep the chicken: Pat the thighs dry and make a few shallow cuts on the meat side. Season both sides with salt and pepper, then dust lightly with cornstarch. Let sit for a few minutes.
Make the sauce: Combine soy sauce, mirin, rice vinegar, oyster sauce, brown sugar, water, and cornstarch in a bowl. Stir until the sugar dissolves and everything is well mixed.
Prep the scallions: Thinly shred the scallions lengthwise and soak in cold water. This keeps them crisp and makes them curl up beautifully — a simple step that makes the dish look restaurant-level.
Pan-fry the chicken: Heat oil over medium heat. Place chicken skin-side down and cook for about 5 minutes until golden. Flip and cook another 5 minutes until cooked through. Set aside.
Infuse the oil with chili: Pour off most of the oil, leaving about 1 tablespoon. Lower the heat and add the dried chilies. Fry briefly until fragrant and the oil turns a light red.
Thicken the sauce: Give the sauce a stir (the cornstarch settles), then pour it into the pan over medium heat. Stir constantly until it bubbles and thickens enough to coat the back of a spatula.
Coat and serve: Return the chicken to the pan and spoon the sauce all over. Cook for 2 more minutes so the glaze clings. Slice, top with drained scallions, and serve immediately.