gently heat the milk with the bay leaf, onion and mace

remove from the heat just before it comes to a boil

cover and set aside to infuse for 30 minutes

strain the milk and discard the flavourings

melt the butter over a low heat

add the flour and stir briskly until it is smooth

you do not want the flour to change colour (this is known as a roux)

gradually stir in the milk

bring to the boil and beat to prevent lumps forming

season and add some nutmeg if desired

simmer for 5 minutes, stirring occasionally