Miso paste
Fermented paste of soybeans, salt and koji, often with rice or barley – salty and savoury, ranging from mild and sweetish to robust depending on the type.
Miso paste is now sold in many larger supermarkets in the Asian section as well as in health food and organic shops, while Asian grocers and online suppliers offer the widest range of types (pale shiro miso, dark aka miso, blended awase miso). Kept chilled and tightly sealed, it lasts for many months.
Substitute:
Korean doenjang comes closest, though it is stronger and coarser in flavour, so use less of it. Soy sauce supplies only salt and umami, not the body and gentle sweetness, so it is no real substitute.