miso
A fermented paste of soybeans, salt and often rice or barley: salty, savoury and deep in flavour.
Miso is sold in Asian grocers, health food shops and increasingly in well-stocked supermarkets, usually as light, mild miso or dark, stronger miso. There is no equivalent substitute, and the usual stopgap is soy sauce.
Substitute:
Soy sauce supplies salt and savouriness, but neither the body nor the rounded, faintly sweet depth of the paste; it will do at a pinch in soups and marinades, but not in glazes and dressings, where miso provides body. Light and dark miso can stand in for each other if you use the dark one more sparingly.