Heat olive oil or butter in a wok or large skillet over medium-high heat.
Add zucchini, mushrooms, onions, and garlic to the skillet.
Sauté for 5-6 minutes, stirring frequently, until vegetables are just tender.
Add bell pepper if using and sauté for another 2 minutes.
Stir in soy sauce, salt, and black pepper.
Cook for 1 additional minute to combine flavors.
Remove from heat and serve immediately.