Place the onion, clove and milk in a saucepan over medium heat and bring to the boil. Remove from the heat and leave to infuse for 30 minutes. Bring back to the boil and strain the hot milk into a jug.
Melt the butter in a heavy-based saucepan over medium–low heat. (I like to use a cast-iron pan, but any pan will do.) Add the flour and cook, stirring with a hea