Heat 1 tablespoon of vegetable oil in a large skillet over medium heat.
Add the chicken pieces and cook until lightly browned and cooked through. Remove from skillet and set aside.
In the same skillet, add the remaining oil, onion, and bell pepper. Sauté for 2-3 minutes until softened.
Stir in the garlic and black pepper, cooking until fragrant.
Return the chicken to the skillet and mix in the soy sauce, oyster sauce, sugar, and chicken stock.
Simmer for 2-3 minutes. If a thicker sauce is desired, mix cornstarch with 1 tablespoon water and stir into the skillet.
Cook until the sauce has thickened. Garnish with green onions before serving.