This is a deliciously flavoured soup and is well worth making when
Jerusalem artichokes are in season from November to June. Heat butter in
a pan and cook onion until soft but not coloured. Add the artichokes to
the pan, pour over the milk and cook slowly for about 7 minutes, shaking
the pan occasionally. Pour the stock into the pan, season, cover and
simmer for about 20 minutes. Allow to cool, then puree to a cream in an
electric blender or pass through a sieve. Return to the pan, taste and
adjust seasoning, re-heat and serve garnished with sprigs of watercress.