1. Heat the butter in a pan. Add the flour and cook for a minute.

2. Gradually add the milk, stirring constantly over the heat until the mixture boils and thickens.

3. Remove from the heat and add the cheese, egg yolks and chopped watercress.

4. Transfer the mixture to a large bowl. Beat the egg white until they form soft peaks, and fold them lightly into the watercress mixture.

5. Pour the mixture into a greased and lined swiss roll tin measuring 25 x 30cms and bake at 375°c / 190°f for 20 minutes or until the roulade is golden brown.

6. Whilst the roulade is cooking, mix together the cream cheese and the sour cream. Stir in the chopped prawns.

7. When the roulade is cooked, quickly turn it out onto a wire rack covered with a tea towel. Carefully remove the lining paper and spread with the cream cheese mixture.

8. Roll up the roulade and spread with the cream cheese mixture. Roll up the roulade using a towel to help roll from the short side.

9. Serve with a home made mayonnaise made with raspberry vinegar, and some crush (and sieved) raspberries lightly folded into the sauce leaving swirls of colour.