For the porcini puree, peel the potatoes, cut into quarters and boil in salted water. Strain the potatoes, puree with a blender and mix with the milk.
Season the puree with salt, pepper and nutmeg, add butter. Clean and chop the porcini mushrooms, sauté in 1 tablespoon butter and stir into the puree.
Serve the porcini puree as a garnish.