The first job is to flavour the milk. We do this by adding the bay leaves, onion and peppercorns plus half the parsley to the milk and simmering it all in a pan for 4 minutes before allowing to cool completely.
To mak the sauce, strain this now deliciously flavoured milk and add the butter and flour stiring with a whisk. Bring to a boil and simmer the sauce as it thickens, whisking continuously until the sauce is thick and glossy.
Finely chop the rest of the parsley and stir into the sauce just before serving.