Cut chicken into bite-sized pieces.
Mix chicken with soy sauce and cornstarch.
Marinate chicken for 10 minutes.
Slice mushrooms and onions evenly.
Heat oil in wok over high heat.
Add marinated chicken to hot wok.
Cook chicken until golden brown.
Remove chicken and set aside.
Add mushrooms to same wok.
Stir-fry mushrooms until slightly softened.
Add garlic and onions to wok.
Cook until onions become translucent.
Return chicken to the wok.
Add black pepper and remaining sauces.
Stir everything together for 2 minutes.
Garnish with green onions and serve.