Prepare the Chicken: Cut the chicken thighs into bite-sized pieces. In a bowl, combine all marinade ingredients except the oil. Add the chicken, ensuring each piece is well-coated. Cover and marinate for 15–20 minutes. Mix in the oil just before cooking.

Make the Black Pepper Sauce: In a separate bowl, mix all sauce ingredients until the cornstarch dissolves completely. Set aside.

Cook the Chicken: Heat a large skillet or wok over medium-high heat. Add a bit of oil and stir-fry the chicken until golden brown and cooked through. Remove the chicken and set aside.

Stir-Fry the Vegetables: In the same skillet, add more oil if needed. Stir-fry the onion, ginger, and garlic for 2 minutes until fragrant. Add the red pepper (or celery) and stir-fry for another 2 minutes.

Combine Chicken and Sauce: Return the chicken to the skillet and pour in the prepared black pepper sauce. Stir well to coat all ingredients evenly. Simmer for 2–3 minutes until the sauce thickens.

Finish with Green Onions: Toss in the green onion pieces and cook for another minute. Adjust the seasoning by adding extra black pepper if needed.

Serve: Serve hot over steamed rice or noodles for a complete and satisfying meal.