Simple cheese sauce

Put the butter into a saucepan and melt until foaming. Stir in the flour and continue to cook and stir over a medium heat for 2 minutes until the mix (roux) feels sandy and smells toasted.

On a medium heat, gradually whisk the cold milk into the roux, in three stages, whisking to remove all lumps between each addition. Once smooth, continue to heat until it comes to the boil, then reduce the heat slightly, and continue to cook, stirring, for 1 minute. The sauce should thickly coat the back of a wooden spoon.

Take the pan off the heat and let the sauce cool slightly. Add the cheese and whisk until smooth. Whisk in the mustard and season to taste.