Ingredients
50g butter 
50g flour 
450ml milk 
½ tsp Dijon mustard (optional) 
150g cheddar cheese, grated 

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.

After 5 minutes add the Dijon if using and then add the grated cheese a third at a time a stir until thoroughly melted.