Toss chicken with salt and pepper. Set aside.
Make sauce by dissolving cornstarch in soy sauce in a small bowl. Then add dry sherry, oyster sauce and sugar; whisk well to combine.
In a wok or large Dutch oven, heat oil over medium high heat. Stir fry chicken until cooked through. Remove to a bowl.
Add more oil if necessary. Stir fry onion and mushrooms for a couple minutes until barely tender. Add garlic; cook 1 minute.
Add zucchini and continue to cook for 1 to 2 minutes more. Be careful not to overcook the zucchini.
Add chicken and char siu; toss to combine.
Add sauce and cook until thickened and heated through.
Serve immediately over rice.