To prepare for this, chop the cucumber into chunks (thankfully no need to peel) and the same with the potato. Slice the spring onions (including the green part), while the lettuce leaves can just be separated and washed, and the leaves torn.
Now in a medium-size saucepan melt the butter and add all the prepared vegetables, stirring them around a bit. Then, keeping the heat low, pop a lid on and let them sweat gently for 10 minutes.
After that pour in the stock, season and, after a couple of stirs, bring it back to a gentle simmer. Put the lid back on so it can simmer gently for another 20 minutes. When the time is up, use a stick blender to purée the soup, making sure there are no escaped bits of cucumber that the blades didn’t catch – it will finish up a magnificent fresh green colour, very pretty.
Reheat gently and serve in warm soup bowls with some cream swirled into each one and a few chives sprinkled over.