We’re not quite sure where or when we first had this, but the inspiration is Vietnamese and it’s one of our all-time favourite soups. The crucial element here is fermented tofu, an ingredient of Chinese origin, which is also much-loved in Vietnam. It smells a bit funky, as any fermented food is wont to do, but it adds an incredible piquancy and burst of umami, and nothing else will really do in its place. It’s cheap to buy and easy to find in Asian supermarkets, and comes in a few guises and flavours. However, you need the red variety here. Fermented tofu lasts for a long time in the refrigerator.