Start by marinating the chicken pieces in a bowl with 1 tablespoon of soy sauce, 1 teaspoon of ground black pepper, and a pinch of salt. Allow it to marinate for at least 10 minutes.
Heat the vegetable oil in a wok or large skillet over medium-high heat.
Add the marinated chicken to the pan, spreading it out evenly. Stir-fry for about 5-7 minutes until the chicken turns golden brown and is cooked through. Remove the chicken from the pan and set aside.
In the same pan, add the sliced onion, minced garlic, and ginger. Sauté for 2-3 minutes until the onions become translucent.
Add the sliced bell pepper to the pan and stir-fry for another 2 minutes.
Return the cooked chicken to the pan and add the remaining soy sauce, oyster sauce (if using), sugar, and the remaining ground black pepper. Stir well to combine everything.
Pour the cornstarch mixture into the pan to thicken the sauce, and cook for another 2-3 minutes while stirring until everything is well coated and heated through.
Taste and adjust seasoning with salt if needed.
Serve hot over cooked rice.