Melt the butter in a saucepan. Add the flour and cook over low heat, stirring with a wooden spoon, for 2 minutes. Do not allow the mixture (roux) to brown.

Remove the pan from the heat and gradually blend in the milk, stirring after each addition to prevent lumps forming. Bring to the boil slowly and continue to cook, stirring all the time, until the sauce comes to the boil and thickens.

Simmer very gently for a further 2 - 3 minutes. stir in the finely grated rind of 1 small lemon and 15 ml (1 tbsp) of lemon juice. season with salt and pepper