Shiitake mushrooms
Savoury brown-capped mushrooms, used fresh or dried; the dried ones are especially aromatic.
Caution: Raw or barely cooked shiitake can cause an itchy skin rash in some people, so they should be cooked through.
Fresh shiitake are sold in many supermarkets, while dried ones are cheaper and more widely available in Asian grocers. Dried shiitake are soaked before cooking, and the soaking liquid makes a savoury stock.
Substitute:
Brown button mushrooms will do at a pinch, but they are milder and softer and give broths and braises far less depth.