Ingredients
50g butter 
50g flour 
450ml milk 
Grating of nutmeg (optional) 

Method
Melt the butter in a small saucepan over a medium heat. When melted add the flour, mix and cook for a minute (this helps to cook out the raw flour taste).

Take off of the heat and slowly add the milk thoroughly mixing as you go (adding the milk slowly stops any lumps forming). Once the milk is incorporated put the sauce back on a low/medium heat and cook for 5 minutes.