A large dark brown mushroom, which is simply a fully mature Cremini. Portabellas are 3-7 days older than Crimini mushrooms when harvested. The name portabella was popularized in the 1980s to sell a then unglamorous mushroom. Because it is harvested later, the portabella can easily reach 5 inches in diameter, and is packed with concentrated flavor. In the matter of portabella versus portobello, both spellings are used. However, the Mushroom Council has adopted the two "a" version to establish some consistency.
Look for a tight underside and light-colored gills. Darkening and loosening in the gills indicates age.
Keep in a brown paper bag in the refrigerator for 7-10 days.
The woody stem is often used in stock-making or cooked as you would other mushrooms. Caps can be chopped, but are more dramatic presented whole, often marinated in balsamic vinegar and grilled in sandwiches as a replacement for meat.