One of Auguste Escoffier's Mother Sauces. This sauce can easily be extended to Mornay (with cheese)Soubise (with onion) and many others
Peel the onion and leave whole. Stud with the cloves and bay leaf. Place the milk in a small saucepan, add the onion. Bring to the boil then take off the heat and leave to infuse for 1 hour.
To make the roux:
Melt the butter in a saucepan, add the flour and cook over a medium heat for 2 min, stirring continuously. Strain the milk into a jug then, add to the roux a little at a time. Make sure you whisk continuously, otherwise the sauce will be lumpy.
Return to the heat and cook very slowly, stirring occasionally for 20 min.
«Whilst still hot, place a sheet of grease proof paper directly on top of the sauce. This will prevent a skin from forming on your sauce.»