Remove the crust from the stale bread and tear the bread into rough chunks around 2cm square.
You should end up with 200g - 225g of cubed bread.
Pour the milk into a saucepan. Press a clove into each quarter of the onion.
Add the onion quarters, peppercorns, bay leaves and the blades of mace (or sprinkle the ground mace into the pan) along with the salt and bring to the mixture almost to boiling point.
Remove the pan from the heat and cover the saucepan with a lid and allow the ingredients infuse for at least half an hour.
After the mixture has infused, strain the mixture and place the saucepan back on a very low heat.
Add the bread to the saucepan and cook for about 10 to 15 minutes, stirring every now and then, by which time the sauce should be thick and warm.
Just before serving, add the butter to the saucepan and stir until it has melted and combined with the sauce. Season to taste with salt and mix in the cream if using.
Grate over the nutmeg and serve.