Melt the butter in a saucepan and fry the onion and potatoes until soft, but not brown, stir frequently. Add the chicken stock and boil for 10 minutes, or until potatoes are soft.
Add the watercress to the boiling mixture and simmer for 1 - 2 minutes.
Remove and blend, add the cream once it is slightly cooled and stir in, adjust the seasoning.
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