Melt the butter and add the leeks (from which you have removed the coarse green parts) and the onion. Allow to soften over a very low heat for about 10 minutes and then season and add the potatoes. Cook gently for a further 5 minutes and then pour on the chicken stock. Leave to simmer for about 45 minutes.
Purée the mixture in a food processor, put through a sieve into a clean bowl and allow to rest in the fridge overnight or for some hours. Before serving, stir in the cream and sprinkle with chives.