In a mixing bowl, combine the sliced chicken, soy sauce, oyster sauce (if using), sugar, and cornstarch. Mix well to coat the chicken. Let it marinate for about 10 minutes.

Heat 2 tablespoons of vegetable oil in a wok or large skillet over medium-high heat. Add the minced garlic and stir-fry for about 30 seconds until fragrant.

Add the marinated chicken to the pan, and cook for 5-7 minutes, stirring frequently until the chicken is browned and cooked through.

Add the sliced bell pepper and onion to the pan. Stir-fry for an additional 3-4 minutes until the vegetables are tender yet still crisp.

Sprinkle in the freshly ground black pepper and add salt to taste. Mix well to combine all the flavors.

Remove the wok from the heat and transfer the Black Pepper Chicken to a serving dish. Garnish with chopped green onions before serving.

For extra heat, you can add sliced fresh chili peppers or a pinch of red pepper flakes during the stir-frying process.

Feel free to substitute or add other vegetables such as broccoli, carrots, or snap peas during the cooking process.

Serve this dish over steamed rice or noodles for a complete meal.