An open-cap mushroom is a mushroom harvested or grown to the stage where the cap has fully opened and flattened, exposing the gills beneath. In commercial terms it usually means large flat mushrooms, the mature form of the common cultivated Agaricus bisporus, sold with caps several centimetres across. Open caps have a deeper, more developed flavour than closed button mushrooms, with a meaty texture that suits grilling, stuffing, and roasting whole. The exposed gills release more moisture during cooking, which concentrates flavour but can darken sauces. Nutritionally they offer B vitamins, selenium, potassium, and, when exposed to sunlight or UV during production, vitamin D2. Portobello mushrooms are essentially the largest open-cap grade, prized as a vegetarian burger and grilling option.