For the bread sauce, stud the onion all over with the cloves, then place into a saucepan along with the bay leaf, peppercorns and milk. Bring the mixture to the boil, remove the pan from the heat and set to cool for 15 minutes.
Remove the onion, bay leaf and peppercorns, then stir in the breadcrumbs.
Return the pan to the heat and cook very gently for 5-10 minutes, stirring occasionally until the breadcrumbs have absorbed all the liquid and the sauce is the desired consistency.
Stir in the butter and season, to taste, with salt and freshly ground black pepper.