Shiitake mushrooms
East Asian mushrooms with a brown cap, used fresh or dried, with a strong, savoury flavour.
Caution: Eaten raw or only briefly heated, shiitake can cause an itchy, streak-like skin rash in some people, so they should always be cooked through.
Fresh shiitake are sold in many supermarkets, while dried ones, often more important in Korean cooking and much stronger in flavour, are reliably found in Asian grocers. Dried shiitake are soaked before cooking, and the soaking water makes a good stock.
Substitute:
Fresh shiitake can be replaced with chestnut mushrooms or king oyster mushrooms, which taste milder. Dried shiitake have no true equivalent; dried porcini also add depth but taste noticeably different, nuttier and less smoky-savoury.