Bring the milk to the boil and then add a quarter of an onion and a few sprigs of fresh thyme. Place your cold roux in a separate pan. Once the milk is boiling, add it to the roux and then return the mixture to the heat. Whisk continually until the mixture thickens and the flour is cooked through. This should take 5-6 minutes and the bechamel is ready when it coats the back of a spoon. Season with salt, pepper and nutmeg and your bechamel is ready to use!