Melt half of the clarified butter in a large pan, fry the mushrooms vigorously while stirring constantly. Season with salt, pepper and nutmeg and remove from the pan.
Melt the remaining clarified butter in the same pan, sauté the shallot and leek briefly, season with salt and pepper, deglaze with vegetable stock and reduce by 2/3. Add the mushrooms, stir in the crème fraîche and heat over a low heat. Add chopped parsley and season again if necessary.