Preheat the oven to 400°F (200°C).
Pat the chicken thighs dry with paper towels. Season both sides with salt and pepper, then lightly dust with 1 tablespoon cornstarch. This helps the glaze stick and adds a slight crust.
Make the orange sauce: In a bowl or measuring cup, whisk together orange juice, zest, soy sauce, honey, brown sugar, rice vinegar, garlic, ginger, and red pepper flakes if using.
Set aside.
Heat a large oven-safe skillet over medium-high. Add the neutral oil. When it shimmers, add the chicken in a single layer.
Sear for 3–4 minutes per side until lightly golden. Do not fully cook through yet. Transfer the chicken to a plate.
Lower the heat to medium.
Pour the orange sauce into the skillet and bring to a simmer, scraping up any browned bits. Simmer 2–3 minutes to let the flavors meld.
Stir the cornstarch slurry (cornstarch + water), then pour it into the simmering sauce. Cook, stirring, for 1–2 minutes until the sauce thickens slightly and turns glossy.
Return the chicken to the skillet, turning to coat in the sauce.
Drizzle in the toasted sesame oil if using. Spoon some sauce over the top of each thigh.
Transfer the skillet to the oven and bake for 10–12 minutes, or until the chicken reaches 165°F (74°C) in the thickest part. The sauce should be sticky and cling to the chicken.
Carefully remove from the oven.
Let the chicken rest 3 minutes, then spoon more sauce over the top. Garnish with green onions, sesame seeds, and an extra sprinkle of orange zest if you like. Serve over rice or noodles.