HEAT PAN: Heat a 10-12 inch skillet over medium heat. Add the butter or oil and allow to just melt.
COOK THE VEGGIES: Add the zucchini, mushrooms, and onions to the hot pan and arrange in an even layer. Allow the vegetables to cook for about 3 minutes before stirring to allow the other sides of vegetables to cook. Stir every couple of minutes for about 6-8 minutes total or until the vegetables become tender.
ADD GARLIC: Add the garlic, salt, and pepper and stir the vegetable for another 30-60 seconds or until the garlic is fragrant.
SERVE OR STORE: Add additional salt and pepper as needed. Serve immediately or allow to cool before refrigerating for up to 5 days.