Melt the butter in a heavy saucepan. When it foams, add the potatoes and onions and turn them until well coated. Sprinkle with salt and freshly-ground pepper. Cover and sweat on a gentle heat for 10 minutes, until soft but not coloured. Add the cucumber and toss it in the butter. Add the stock, bring to the boil and cook until the potatoes, onions and cucumbers are completely cooked. Add the lettuce and boil with the lid off for about 4–5 minutes, until the lettuce is cooked. Do not overcook or the soup will lose its fresh green colour. Add the mint and cream if using, and liquidise. Taste and correct seasoning if necessary. Serve in warm bowls garnished with a blob of whipped cream and a little freshly-chopped mint.
© 1989 Darina Allen. All rights reserved.