Béchamel sauce

Put the milk in a saucepan with the bay leaf, parsley stalks (if using) and peppercorns and bring to a simmer. Strain into a jug and rinse the pan out.
Put the pan back on the heat and gently melt the butter, stir in the flour (use a wooden spoon or mini whisk) and cook for 1 minute, don’t let the butter or flour brown at all.
Take the pan off the heat and add the milk in 3 stages. At each stage beat or whisk the milk into the flour until it is completely smooth.

Once the flour is all in, bring the mixture back up to a simmer, stirring all the time to stop any lumps forming. Once the mixture looks as if it won’t thicken any more, simmer it for 5 minutes, stirring/whisking now and then, until it looks really glossy.
Season with salt and white pepper (or black if you don’t mind the speckles). If you do have a few lumps, just strain the lot through a sieve.