A chilled leek and potato soup, often tinned, always excessively creamy, featured on virtually every summer menu in mid-1970s Soho, but its origins go back much further. Louis Diat, the chef at New York’s Ritz Carlton Hotel, claimed to have invented it in the 1930s saying, as all French chefs do, that it was inspired by his mother’s Potage bonne femme. Anywhere other than the States it could only be made in winter, as leeks were, and remain to this day, a strictly winter ve