In a large mixing bowl, combine the cut chicken pieces with 1 tbsp soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, and 1/4 tsp white pepper. Mix well to ensure all chicken pieces are coated. Let it marinate for at least 15 minutes at room temperature, or up to 30 minutes in the refrigerator.

In a small mixing bowl, whisk together 3 tbsp soy sauce, 2 tbsp oyster sauce, 1 tbsp granulated sugar, 1/2 cup water (or chicken broth), 1 tbsp cornstarch, and 2 tbsp freshly ground black pepper until smooth. Set aside.

Heat 1 tbsp vegetable oil in a wok or large skillet over high heat until shimmering. Add the marinated chicken in a single layer, ensuring not to overcrowd the pan (cook in batches if necessary). Stir-fry for 3-4 minutes until the chicken is cooked through and lightly browned. Remove the chicken from the wok and set aside.

Add the remaining 1 tbsp vegetable oil to the wok. Add the minced garlic and grated ginger, and stir-fry for about 30 seconds until fragrant.

Add the sliced yellow onion, green bell pepper, and red bell pepper to the wok. Stir-fry for 2-3 minutes until the vegetables are slightly tender-crisp.

Return the cooked chicken to the wok with the vegetables. Give the prepared black pepper sauce a quick whisk again, then pour it over the chicken and vegetables.

Stir continuously for 1-2 minutes until the sauce thickens and coats all the ingredients evenly. Remove from heat. Garnish with sliced green onions and an extra sprinkle of fresh black pepper if desired.

Serve immediately with steamed white rice or noodles.