The Portabella mushrooms are much larger then white or cremini mushrooms. The caps are open and flat and can measure up to 6-inches in diameter. You can see that underneath, the gills are exposed which means that some of the mushrooms moisture has evaporated, the reduced moisture concentrates and enriches the flavor of the mushroom and creates a dense, meaty texture. This is why often times you will see people use Portabella mushrooms being used as a meat substitute. They are great on the grill and make a delicious burger or vegetarian alternative for steak. Even meat lovers enjoy their hearty texture!
The stems tend to be tough and woody, so most often they are removed but be sure not to toss them because they are great tossed into soups and stocks.