Melt the butter in a pan, then add the flour. Cook for 1-2 minutes, stirring with a wooden spoon. This is the roux which is the basis of the sauce.
40 g butter, 40 g plain flour (all-purpose flour)
Add about a fifth of the milk to the pan, whisk well and it will become incorporated with the roux, and thicken up.
500 ml semi-skimmed milk
Add another lot of milk, and again whisk it in well.
Continue, adding the milk in about 3 more lots, and whisking after each addition and allowing it to thicken.
Set the pan to cook gently for around five minutes, it should be at a simmer. Stir occasionally. Around the end of the cooking time, the sauce will thicken, make sure you stir it well and whisk during this time.
Once thickened your white sauce is ready to use. Note it will get very thick, and you want it to get very thick before adding it into the slow cooker.