Cover the porcini mushrooms with boiling water and soak for 20 minutes. Drain, reserve the soaking liquor and squeeze out excess liquid. Chop and set aside. Meanwhile, pour boiling water over the lasagne sheets and soak for 2 minutes. Drain and spread out in a single layer on paper towels or clean tea towels (dish towels).
Heat the oil or butter in a frying pan and add the fresh mushrooms, the porcini mushrooms and garlic. Cook, stirring, until the mushrooms are tender and releasing their juices. Add the squeeze of lemon juice,
To make the béchamel, bring the milk to the boil. Meanwhile, stir the butter and flour together in a pan over a medium heat. Remove from the heat and gradually add the hot milk, stirring constantly, making sure each addition is absorbed before adding the next. Return to the heat, add the nutmeg and season with salt and the white pepper. Cook, stirring, until the sauce is the consistency of thin custard. Add