Simple to make, incredibly versatile and freezes brilliantly.
Heat the 2L Saucepan over medium heat for 2 minutes.
Add butter. When melted, add flour and stir well.
Season with salt and pepper and cook, whisking constantly, for 2 minutes.
Add cold milk and mustard and continue to whisk until the mixture comes to the boil and thickens. Cook for a further 3 minutes.
Add cheese and whisk through.
If sauce is too thick, add a little more milk. Consistency should be of thick custard.