Slowly bring the milk to the boil with the bay leaf and the onion. Slowly, because we wish to infuse the milk with the bay and onion flavours.
Melt the butter in another pan (it must be big enough to comfortably hold the milk), then add the flour. Stir in to make a roux by cooking over a medium heat, stirring continuously with a wooden spoon for 10 minutes. Slowly start to add the strai