The Portobello mushroom is a large brown strain of the common white mushroom (Agaricus bisporis). Portobello mushrooms are distinguished by their large size, thick cap and stem, and a distinctive musky smell. Because of their size - and the thickness of their fleshy caps - these mushrooms can be cooked in a range of different ways, including grilling and frying. They are also known as the portobella mushroom.