This makes a superb vegetarian treat if you substitute vegetable stock for the chicken stock. Fresh wild mushrooms have an exquisite flavour but if necessary you could replace some of them with reconstituted dried mushrooms. Occasionally I like to top toasted brioche with this mixture.
Clean the mushrooms by wiping them with a damp cloth. Separate the porcini stems from the caps, removing the soft underpart of the cap if it is too spongy. Peel the stems and cut into slices 5mm (¼ in) thick. Cut the caps into large pieces, depending on their size.
Heat half the olive oil in a frying pan, add the garlic and cook for 4-5 minutes, until lightly golden. Add the porcini stems and sauté for a minute or two. Add the Madeira and chicken stock and cook for 5 minutes. Transfer to a blender and blitz until smooth. Pour into a saucepan and keep warm.
Wipe out the frying pan and heat the remaining olive oil in it over a high heat. Add the porcini caps and chanterelles and sauté for 5 minutes, then add the leaves from the thyme sprigs (reserving some for garnish) and the prunes and cook for 2-3 minutes. Stir in the walnuts.
Reheat the sauce but do not let it boil. Add the truffle oil, if using, and then whisk in the butter a few pieces at a time. Season to taste. Pile the mushrooms and prunes on to individual serving plates and surround with the sauce. Garnish with a little fresh thyme and serve immediately.