Mix the sauce: In a small bowl or measuring jug, combine the soy sauce, apple juice, brown sugar, rice vinegar, grated garlic, and grated ginger. In a separate small bowl, whisk the cornstarch and water until completely smooth, then pour the slurry into the soy sauce mixture and stir until the sugar begins to dissolve. Set aside; stir again right before it goes into the pan, as the cornstarch settles within 5 minutes.
Dry and season the chicken: Pat the chicken thighs completely dry on both sides with paper towels - surface moisture steams instead of searing and prevents caramelization. Season both sides lightly with a small pinch of salt; the sauce carries most of the seasoning.
Sear the chicken: Heat the neutral oil in a cast iron skillet over medium-high heat until it shimmers. Place the thighs smooth side down without crowding (sear in two batches if needed) and cook undisturbed for 5 to 6 minutes, until the underside is deep golden-amber. Flip and cook the second side for 4 to 5 minutes. The chicken finishes cooking in the sauce.
Add the sauce: Reduce the heat to medium. Stir the sauce to redistribute the cornstarch, then pour it carefully around the chicken pieces rather than directly over them. It will bubble immediately around the edges; tilt the pan to distribute it evenly.
Glaze and finish: Cook the chicken in the simmering sauce for 3 to 4 minutes, spooning sauce over each piece every 60 seconds. The glaze is ready when a spoon dragged across the pan base leaves a trail that fills in slowly over 3 to 4 seconds. Check the thickest piece reaches at least 74°C (165°F), then remove from heat.
Rest and slice: Transfer the glazed chicken to a cutting board and rest for 5 minutes so the juices redistribute. Slice on a diagonal into 1.5cm strips and serve over rice with the pan sauce drizzled over the top.
