100g strong cheddar (or 50g cheddar, 50g blue cheese)
Pour the milk into a large saucepan and add the flour and 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 the cheese (or cheeses) until melted.