Pour cooking oil into a wok or large skillet. (Add more oil as necessary during cooking.) Preheat over medium-high heat. Stir-fry zucchini and onion in hot oil for 3 to 4 minutes or until crisp-tender. Remove zucchini mixture from the wok.
Add mushrooms and sweet pepper to hot wok. Stir-fry for 1 to 1-1/2 minutes or until crisp-tender. Remove mushroom mixture from wok.
Add bean sprouts, soy sauce, sugar, and pepper to the hot wok. Stir-fry for 1 to 2 minutes or until bean sprouts are crisp-tender. Return the zucchini mixture and the mushroom mixture to the wok. Cook and stir gently until heated through. Serve immediately. Makes 4 servings.