In a large resealable plastic bag, combine soy sauce and brown sugar; add the chicken. Seal bag and turn to coat; refrigerate for 20 to 30 minutes.
In a large nonstick skillet or wok, stir-fry onions and garlic in oil until tender. Remove with a slotted spoon to a bowl. Add celery, mushrooms and green pepper to skillet; stir-fry 3 to 4 minutes or until crisp-tender. Remove with slotted spoon to bowl. Add chicken and marinade to skillet and stir-fry for 5 to 7 minutes or until chicken is no long pink. Return vegetables to skillet.
Combine cornstarch and water until smooth and stir into chicken mixture. Bring to a boil; cook and stir for 2 minutes or until thickened. Add the bean sprouts, salt and pepper; cook and stir for 2 minutes or until heated through. Serve over rice if desired.
Recipe by Alberta Chicken Producers Recipes and Industry Information at https://chicken.ab.ca/chicken-chop-suey/