Marinate the Chicken: Combine chicken with 3 tbsp water, 1 tsp cornstarch, 1 tsp oil, 2 tsp oyster sauce, and ½ tsp black pepper. Set aside.
Make the Sauce: Mix chicken stock, oyster sauce, soy sauces, sugar, sesame oil, and set aside.
Sear Chicken: In a hot wok, heat 1 tbsp oil and stir-fry marinated chicken until browned and cooked. Remove and set aside.
Sauté Aromatics: Add 1 tbsp oil, sauté chopped garlic and onions until fragrant.
Deglaze: Add Shaoxing wine and stir for a few seconds.
Add Veggies: Toss in bell peppers and celery, stir-fry briefly.
Combine Everything: Return chicken to the wok, pour in the sauce, and stir well.
Thicken: Add cornstarch slurry and stir until sauce thickens. Serve hot.