Heat the milk with the onion (studded with a bay leaf and clove). Leave to infuse for 20 minutes then drain. Meanwhile in a clean pan melt the butter and add the flour. Cook for 1 minute, stirring constantly.
Gradually beat in the milk very slowly and continue stirring as the sauce becomes thicker. Once all the milk has been added and the sauce has thickened, the cheese can be sprinkled over the sauce and stirred in.
