Pour 500ml milk into a large saucepan and add 4 tbsp plain flour and 50g butter.

Turn on the heat to medium and start to whisk the mixture. Keep whisking fast as the butter melts and the mixture comes to the boil – the flour will disappear and the sauce will begin to thicken.

Whisk for another 2 mins while the sauce bubbles then stir in 100g grated strong cheddar cheese (or 50g grated cheddar, 50g crumbled blue cheese) until melted.