In a small bowl, whisk together the chicken broth, soy sauce, oyster sauce, cornstarch, sesame oil, sugar, and black pepper. Set the sauce aside.
Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat.
Add the sliced chicken and cook until lightly browned and cooked through. Remove from the pan and set aside.
Add the remaining oil to the pan.
Stir fry the onion, garlic, carrots, celery, cabbage, and bell pepper for 4 to 5 minutes until slightly tender but still crisp.
Add the bean sprouts and cook for 1 more minute.
Return the cooked chicken to the skillet.
Give the sauce another quick stir, then pour it into the pan.
Cook for 2 to 3 minutes, stirring constantly, until the sauce thickens and coats the meat and vegetables.
Serve hot over steamed rice or noodles.