This is the same species as flat and button mushrooms, Agaricus bisporus. Swiss browns were the original type of Agaricus bisporus overtaken by the popularity of a mutant strain of the species which was white in colour, now sold as button and flat mushrooms. Swiss browns have a robust flavour and are sold as Portobello mushrooms in their more mature stage. They are a excellant all round mushroom. Avoid withered, slimy, pitted, or bruised mushrooms. Mushrooms are mostly grown inside and as a result quality is consistent all year round. However, mushrooms donâ€™t like the heat so quality can be diminished in summer.