The golden rule here is to use cold milk. So if you want to infuse the milk, pour it into a bowl and let it get completely cold before you begin.
Makes about 15 fl oz (425 ml)
Ingredients 15 fl oz (425 ml) milk this can be infused but must be cold
¾ oz (20 g) plain flour
1½ oz (40 g) butter
salt and freshly milled black pepper
When you're ready to make the sauce, put the milk in a saucepan, then simply add the flour and butter and bring everything gradually up to simmering point, whisking continuously with a balloon whisk, until the sauce has thickened and becomes smooth and glossy.
Then turn the heat down to its lowest possible setting and let the sauce cook very gently for 5 minutes, stirring from time to time. Meanwhile, taste and add seasoning.