Using an immersion blender, combine 3/4 of the porcini with the egg and mascarpone. Then mix in the rest of the sliced mushrooms, parmesan, parsley, salt and pepper.
Butter four small aluminium moulds, pour in the mixture and cook for 40 minutes at 140°C in a bain marie.
Allow to cool slightly then turn out onto a plate.
Cover with the cheese sauce, garnish to taste and serve.