1. Stud the onion halves with the cloves and put in a saucepan with the milk, mace, bay leaves, peppercorns and salt. Heat to just bubbling, then set aside to cool and infuse.
2. Strain the infused milk into a clean pan and gently reheat. Whizz the bread in a food processor to crumbs, then add to the milk, stirring until it thickens. Season, grate in a little nutmeg to taste and add more milk if it looks too thick; keep warm.
3. For the brown butter, melt the butter in a small pan over a low heat, swirling every now and again, until it turns golden brown and smells nutty – about 5-8 minutes. Add the chopped rosemary and cook for 1 minute. Take off the heat, add a pinch of salt and spoon over the bread sauce before serving.