Melt the butter in a heavy saucepan. When it foams, add the onions and potatoes, and turn them until well coated. Sprinkle with salt and pepper. Cover, and sweat on a gentle heat for 10 minutes. Add the chopped lettuce leaves and stock. Boil until just tender, but do not overcook or the lettuce will lose its colour and flavour. Make a purée in a blender or food processor (after cooling slightly first), then sieve and add the mint, the cream, if using it, and more salt and pepper, if necessary. Bring back to the boil and serve immediately.
© 1995 Frances Bissell. All rights reserved.