Make the Sauce: In a small bowl, whisk together broth, oyster sauce, rice vinegar, garlic, sugar, cornstarch, black pepper, chili powder, and ginger powder. Set aside.
Prep the Chicken: Toss chicken thighs in a bowl with 1 tbsp cornstarch and a splash of soy sauce. Let it sit for 10 minutes.
Sear the Chicken: Heat oil in a skillet over medium-high. Sauté chicken until browned and half-cooked. Remove and set aside.
Cook Veggies: In the same pan, add more oil if needed. Stir-fry bell pepper, onion, and celery for 5–7 minutes until slightly tender.
Combine: Return chicken to the skillet. Add sauce and cook for another 3–4 minutes until sauce thickens and chicken is fully cooked.
Serve: Spoon over rice or noodles. Garnish with extra cracked pepper or green onions if desired.