Pour hot water over the dried mushrooms in a small bowl and leave to rehydrate.
Make a béchamel sauce: melt the butter in a heavy pan and stir in the flour. Cook gently, stirring, for about 1 minute, then add the milk and cook, stirring, until thick and smooth. Season with salt, pepper and nutmeg. Cook gently for 20 minutes, stirring regularly.
Rinse the plumped dried mushrooms, then chop them coarsely. Sauté them with the fresh mushrooms in the butter over a high heat. When done, add the garlic, stir in and cook for 2 minutes. Stir in the parsley and season with salt and pepper.
Brush the sides and base of a rectangular 25 × 20-cm / 10 × 8-inch ovenproof dish with olive oil. Spoon in a layer of sauce, lay 4 sheets of the pasta on top and cover with a layer of mushrooms with their liquid from the pan. Lay another 4 sheets of pasta on top. Cover with sauce and grated Parmesan. Add another layer of mushrooms, a final layer of lasagne and finish with a surface coating of béchamel sauce. The pie can be left to stand for an hour or so before cooking, or can be refrigerated overnight.
Bake in an oven preheated to 190°C/375°F/gas 5 for 45 minutes or as instructed on the pasta packet.
© 1998 Alastair Little and Richard Whittington estate. All rights reserved.