In a bowl, marinate chicken with soy sauce, oyster sauce, and cornstarch. Set aside for 15 minutes.
Heat oil in a wok or skillet over medium heat. Add marinated chicken and cook until golden brown. Remove and set aside.
In the same pan, stir-fry bell pepper and onion until tender-crisp.
Add black pepper, sugar, and chicken stock to the vegetables. Stir to combine.
Return the chicken to the pan and toss to coat in the sauce.
Sprinkle with green onions before serving. Enjoy hot!