Portabella mushrooms have become increasing popular over the years, and are actually mature Crimini mushrooms that have a meatier flavor. When picking a good portabella, the first checkpoint is to look for a firm cap and stem. Shriveled or soft caps should be avoided. Turn the mushroom over and take a look at the gill structure. It should be dry, with an ever so slight pinkish tone to it, when viewed in the light. If it's deep dark black or wet looking, the mushroom is past its prime. Portabellas are delicious grilled, roasted or stir-fried.