Melt half the butter in a saucepan over a low heat. Add the flour and cook, stirring with a wooden spoon, for 1–2 minutes until a pale biscuity colour. Remove from the heat and gradually stir in the milk until smooth. Return to a medium heat and cook, stirring constantly, until thick and velvety. Add the nutmeg and season with salt and pepper, then stir in the parsley and 2 tablespoons of the Parmesan. Remove from the heat and leave to cool to room temperature.
Melt the remaining butter with the oil in a large, heavy-based frying pan. Add the fresh mushrooms and cook over a high heat for 2 minutes. Stir in the garlic and cook for 1 minute. Season with salt and pepper. Pour in the wine and porcini and their soaking water. Cook, stirring, until the liquid has evaporated. Stir into the white sauce.
Grease an ovenproof dish, about 19 x 30 cm (8 x 12 inches). Cover the base with a layer of slightly overlapping lasagne sheets. Top with a quarter of the sauce, then continue layering, finishing with a layer of sauce. Scatter with the remaining Parmesan. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes. Drizzle lightly with truffle oil and serve with a green salad, if liked.