Boil the pieces of lasagne about 4 at a time in plenty of boiling salted water with a tablespoon of oil stirred into it. They take about 10–15 minutes. As each batch is cooked lay the pieces on a clean cloth to drain. Add the grated cheese (not the Parmesan), the crushed garlic, the nutmeg, salt and black pepper to the white sauce and heat until the cheese has completely melted. Take off the heat and stir in thoroughly the lightly whisked egg. Reserve ½ pint (275 ml) or a little more of this sauce for the top. Stir the cooked chopped chicken and sliced mushrooms into the remaining sauce. Butter a large shallow ovenproof dish and arrange a layer of lasagne pieces on the bottom. Cover this with a layer of the chicken, mushroom and sauce mixture, then another layer of lasagne and so on, ending with a layer of lasagne. Spread the remaining cheese sauce on top of this, sprinkle liberally with grated Parmesan and dot with butter. If you have prepared it in advance, put the dish in the fridge until you want to cook it. Bake in the centre of the oven at Gas 4 (350°F/180°C) for about 40 minutes until golden brown on top. Serve it with a tomato and crisp lettuce salad.