A good summer soup hot or cold, but if served cold reduce the amount of potato. Use lettuces that remain underdeveloped in the garden, the leaves of ones that have just bolted or the outside leaves of about three lettuces.
Melt the butter and stew the onion and potato gently, covered, for twenty minutes without browning. Add the stock and bring to the boil then add the lettuce, mange tout if using, and choice of herbs, season to taste, and simmer for ten minutes. Cool slightly then liquidize. Reheat before serving adding most of the cream without allowing the soup to boil. Serve with a swirl of cream and a sprinkling of chives or chervil. This soup freezes very well so can be made in larger batches. Leave out the cream if freezing and add when it has defrosted and is being reheated.