Remove the crust from the bread and tear the stripped loaf into a mound of rough chunks or cubes about 2cm/¾in in size. You should end up with 175-200g (6¼-7¼oz) of cubes. If the bread is not slightly stale already, leave the pieces out on a wire rack to dry out.
Pour the milk into a saucepan. Press a clove into each quarter of the onion.
Add the onion quarters, bay leaves, peppercorns and the blades of mace (or sprinkle the ground mace into the pan) along with the salt and bring to the mixture almost to its boiling point.
Remove the pan from the heat. Cover the pan with a lid and let the ingredients infuse for at least half an hour, though you can leave it for a few hours if that helps with your cooking schedule.
After the mixture has infused, place the pan back on a very low heat. Using a slotted spoon, remove the onions, peppercorns, cloves, bay leaves and the blades of mace.
Add the bread to the saucepan and cook for about 15 minutes, stirring every now and then, by which time the sauce should have become thick and warm.
Just before serving the bread sauce, add the butter to the saucepan and stir until the butter has melted and combined with the sauce and season, to taste, with salt.
Add the cream (if using). Grate over quite a bit of nutmeg, adding more once you have poured the bread sauce into a warmed bowl or gravy boat.