Pour the milk into a medium-sized saucepan, add the onion halves, cloves, peppercorns, mace (or nutmeg) and salt and heat over a medium heat, stirring well, until the milk starts to simmer. (NB: Do not allow the milk to boil.)
Continue to simmer the milk, without boiling, for 4-5 minutes, then remove the pan from the heat and set aside, covered, for one hour to allow the flavours to infuse.
when the milk for the bread sauce has infused, add the breadcrumbs to a clean saucepan and strain the infused milk over the breadcrumbs through a fine sieve to collect the onion and spices. Stir once and set aside to soak for five minutes.
Heat the bread mixture over a medium heat, stirring regularly, for 8-10 minutes, or until the bread has absorbed most of the liquid and the mixture has thickened, then remove the pan from the heat and stir in the butter until the butter has melted and the bread sauce resembles porridge. (If the sauce is too thick add a little more milk to loosen it.)
