Heat the butter in a large pan over a medium-low heat and add the onion. Season and cook gently until soft, but not coloured. Stir in the flour and cook for a minute
Gradually stir in the milk and 250 mls water. Bring to just below the boil, cover and simmer for 5 minutes
Meanwhile, prepare a large bowl of iced water. Bring a large pan of water to the boil and plunge in the watercress to wilt, reserving a few leaves to garnish. Tip out into a colander, and then into the iced water
Squeeze the cooled watercress out and add to the pan Puree the whole lot thoroughly, add the cream and season to taste