Heat the oven to 200ºC/180ºC fan/Gas Mark 6.
Put the squash in a roasting tin, drizzle with 1 tbsp of olive oil and sprinkle over the nutmeg. Roast for 30 minutes until soft.
Meanwhile, heat the remaining 1tbsp of olive oil in a frying pan and fry the onions for 3 minutes over a gentle heat, then add the garlic and cook for 2 more minutes.
Add the mushrooms, and cook for 5 minutes to soften. Spoon in the crème fraîche and pour in the stock, stir and simmer for 15 minutes until the liquid is reduced to a creamy consistency. Season.
Pierce a couple of holes in the bag of spinach and microwave according to the pack instructions. Alternatively, empty into a pan, cover and cook until wilted. Drain in a colander and press to remove excess liquid. Crush the squash with a potato masher.
Spread a thin layer of the mushroom sauce over the base of a large, shallow, ovenproof dish (30x20cm/12x8in) and top with 2 sheets of lasagne. Spoon half the squash over the lasagne. Sprinkle over half the spinach, then half of the remaining mushroom sauce. Top with 2 more sheets of lasagne and repeat the squash and spinach layer. Top with 2 more lasagne sheets and a final layer of mushroom sauce. Sprinkle over both the cheeses.
Cover with greased foil and bake for 20 minutes. Remove the foil and cook for a further 30 minutes, until the topping is golden and bubbling. Serve immediately.
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