Stud the onion with the cloves and place in a saucepan with the bay leaf, milk, and a good pinch of nutmeg.
Heat the milk to just below boiling point.
Remove from the heatand leave to infuse for a minimum of 30 minutes before discarding the onion,cloves and bay leaf.
Drain the porcini mushrooms and chop finely, but do not discard the water.
In a saucepan, fry the chopped porcini in 25g of butter.
Allow them to cook slowly for about 8 minutes, stirring continuously.
Add the infused milk, and stir in the cream, sourdough crumbs and half of the porcini water.
Cook on a very low heat stirring until the sauce has thickened.
Season to taste and add a little more of the porcini water if the sauce is too thick.