**Marinate the Chicken:** In a mixing bowl, toss the chicken pieces with cornstarch and soy sauce. Let it marinate for 10-15 minutes while preparing the other ingredients.
**Prepare the Sauce:** In a small bowl, whisk together soy sauce, oyster sauce, hoisin sauce, sesame oil, sugar, black pepper, and water. Set aside.
**Cook the Chicken:** Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook for 4-5 minutes until golden and cooked through. Remove the chicken from the skillet and set aside.
**Sauté the Vegetables:** Add the remaining tablespoon of oil to the skillet. Stir-fry the onion, bell pepper, celery, garlic, and ginger (if using) for 3-4 minutes until slightly softened but still crisp.
**Combine Everything:** Return the chicken to the skillet and pour the sauce over the mixture. Stir well to coat everything evenly. Cook for 2-3 minutes, allowing the sauce to thicken slightly.
**Serve:** Serve hot over steamed rice or noodles, garnished with green onions if desired.