Kombu
Dried kelp, a brown seaweed sold in stiff, dark green sheets; the foundation of the Japanese stock dashi.
Caution: Kombu is very high in iodine, so it is best used mainly for making stock rather than eaten in large amounts, especially by people with thyroid conditions.
Kombu is dried kelp sold in thick, leathery sheets or strips, often dusted with a white, salty bloom that is part of the flavour and should not be washed off, at most lightly wiped. It is rarely eaten on its own; instead it is soaked in cold water or gently heated to draw out its flavour, which is deep, rounded and savoury with a light taste of the sea, carried by plenty of natural glutamate. If it is boiled hard the stock turns slimy and bitter, so it is removed just before the water simmers. Cooked along with beans or rice it makes them more savoury, and soaked leftover pieces can be sliced and braised. Kombu is found in Asian grocery stores, often in health food and organic shops, and by mail order; kept dry and sealed it lasts a very long time.
Substitute:
There is no true substitute. A pinch of MSG supplies the savouriness but not the taste of the sea or the depth; instant dashi granules come close in flavour but usually contain fish, which rules them out for vegetarian dishes.