Melt the butter in a large pan over a medium-low heat. Add the leeks or onion, cover and sweat gently for about 10 minutes, until soft.
Add the potato and stock. Bring to the boil, then simmer for about 10 minutes, until the potato is almost cooked. Add the cubed cucumbers and shredded lettuce, return to the boil and simmer for a further 4 minutes.
Fish out the potato chunks and rub them through a sieve, mouli or potato ricer into a large bowl (whizzing them in a blender would make the soup gluey). Purée everything else in a blender, add to the sieved potato and stir well. Stir in the cream or crème fraîche and season with salt and pepper. Leave to cool completely, then chill for a couple of hours.
Meanwhile, make the croûtons. Heat the oil in a frying pan over a medium-high heat. Add the bread and fry, turning often, for a few minutes, until golden brown. Leave to cool.
Serve the chilled soup topped with a swirl of cream or crème fraîche, chopped chives and the croûtons.
chunks
to finish
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