Pick over and destalk the spinach, then blanch in rapidly boiling salted water for 60 seconds. Refresh in cold water, drain and squeeze out all the moisture you can.
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. Cook gently for 20 minutes, stirring regularly. Stir the grated cheese into the sauce and cook for a further minute or two. Season with grated nutmeg, salt and pepper and remove from the heat.
Butter a suitably sized ovenproof dish and coat the base with a few spoonfuls of the cheese sauce. Cover this with a single layer of oven-ready lasagne. Cover this with spinach and a few spoonfuls of sauce. Repeat with another layer of lasagne and the rest of the spinach and finally a layer of lasagne topped with the rest of the cheese sauce.
Bake at 180°C/350°F/gas 4 for 30 minutes, or as the pasta packet instructs, when the top should be golden and bubbling hot.
© 1998 Alastair Little and Richard Whittington estate. All rights reserved.