Portabella mushrooms add delicious flavor to a variety of dishes, and can also be used as a meat substitute. Learn how to prepare portabella mushrooms for meals and as a veggie burger. Portabella mushrooms are commonly found at the grocery store as caps or as slices. When you get home, take the mushrooms out of the plastic, and keep them in the fridge wrapped in paper towels. When you are ready to cook them, quickly rinse or wipe off portabellas with a squeezed damp towel. The portabella mushroom is a versatile vegetable for all kinds of cooking.