Pick through the watercress to remove any yellowing leaves and wash well. Blend the cooked spinach and the watercress together in a food processor or blender for at least 5 minutes until really fine. If you find that the watercress is a little too dry to go round the machine, add about 25 ml/1 fl oz of the fish stock just to dampen it. If you add too much, however, it will go round but not purée.
Bring the fish stock to the boil along with the white wine and reduce over a high heat until only a quarter of the liquid remains. Add the cream and return to the boil. Just as the sauce starts to thicken, stir in the watercress and spinach purée. Allow the sauce to simmer for a minute, then season with the lemon juice, salt and pepper to taste. Pass the sauce through muslin or a fine strainer, and serve immediately.