Marinate the Chicken: In a bowl, combine the chicken pieces, soy sauce, and cornstarch. Mix well and let it marinate for at least 15-20 minutes.
Heat the Oil: In a large skillet or wok, heat the vegetable oil over medium-high heat.
Cook the Chicken: Add the marinated chicken to the skillet and cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.
Sauté the Vegetables: In the same skillet, add the sliced mushrooms, bell pepper, onion, garlic, and ginger. Stir-fry for about 4-5 minutes, or until the vegetables are tender.
Combine Chicken and Vegetables: Return the cooked chicken to the skillet. Sprinkle in the black pepper and stir to combine. If using, add the oyster sauce and mix well. Cook for an additional 2-3 minutes to heat through.
Serve: Plate the black pepper chicken with mushrooms and garnish with chopped green onions. Serve hot over rice or noodles if desired.
