Stick the cloves into the onion. Put the milk into a saucepan and add the onion, garlic, mace, bay leaf, peppercorns and allspice. Simmer the milk and spices for 15 minutes and leave to infuse for 1 hour.
Strain the milk over the breadcrumbs, add the butter and salt, and stir into the mixture. Return the sauce to the heat and cook over a gentle heat for 15 minutes, stirring occasionally. Add the cream, check the seasoning and serve at once.