Melt the butter in a saucepan and add the onion. Fry gently for about 5 minutes or until the onion is soft and tender, but not brown. Peel and dice the potato and add to the pan. Toss in the onion-flavoured butter and then stir in the stock. Bring up to the boil, lower the heat and simmer for about 15 minutes or until the potato is quite tender.
Meanwhile separate the lettuce leaves, wash them and shred coarsely. You can also use the coarser outer leaves of several lettuces which are to be used for salad, or 2–3 smaller lettuces. Either way the amount should be the equivalent of a good-sized lettuce.
Add the lettuce to the pan, bring back to the boil and then draw off the heat. Ladle the liquid and vegetables a little at a time into a blender and purée. Pour into a bowl, check the seasoning and allow to cool. Chill until ready to serve. It might be necessary to thin the cold soup down with a little extra stock or cream to get the right consistency. Stir in the chopped chives and serve.