In a saucepan or microwave gently heat the milk( put the bay leaf to infuse in the milk)

Put the butter into another saucepan and melt over a gentle heat

Add in the flour and stir, allow the mix to cook out until it has gone a straw like colour

Gradually add in the warm milk , whisking between each addition

add the salt and pepper and mustard if using

Keep stirring until the mixture has thickened

This may seem that it will never happen but just keep stirring

Now add in your cheese or parsley if using

Or omit the above step and use on your lasagne