In a small bowl, whisk the cornstarch, water and soy sauce until smooth; set aside. In a large skillet or wok, stir-fry the broccoli, carrot and onion in oil for 5 minutes. Add the cabbage, zucchini, mushrooms and garlic. Stir-fry until vegetables are tender. Stir soy sauce mixture; add to skillet. Cook and stir until thickened. Serve with rice if desired.