Peel onion, cut in half and stud with clove and bay leaf, place in small pot and cover with the milk. Bring to the boil then take off the heat and leave to infuse for 1 hour.
Melt the butter in a saucepan, add the flour and cook over a medium heat for 2 min, stirring constantly. 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.
«Any flavours can be added to the cold milk before bringing to the boil. These will be infused in to the final sauce.»