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Heat the oil in a large casserole dish. Add the onions and cook until softened, then add the garlic and cook for a few mins more. Push the onions and garlic to the edge of the pan, add the chicken breasts and cook for a few mins each side to brown. Sprinkle over the flour and stir it into the onion mixture. Tip in the stock and season. Cover and simmer for 10 mins – or until the chicken is cooked through. Using 2 forks, shred the chicken into bite-sized chunks. Add half the mascarpone and the pesto, stirring until the mascarpone has melted into the sauce, bubble for a few mins to thicken a little, then remove from the heat and stir in the basil.
Meanwhile, place the squash in a large bowl with 2tbsp of water, cover with cling film and microwave on High for 12-15 mins, until really soft. Season the squash with a little of the nutmeg and some salt and pepper, and mash lightly with a fork.
Heat oven to 180C/160C fan/gas 4. Spoon half the chicken mixture into the bottom of a 20 x 30cm ovenproof dish. Cover with 4 lasagne sheets, then the butternut squash and half the Parmesan. Top with another 4 lasagne sheets, the remaining chicken mixture, then a final layer of lasagne. Mix enough milk into the remaining mascarpone to make a smooth sauce – you need to aim for a consistency similar to white sauce. Season with remaining nutmeg, salt and pepper, then pour over the lasagne. Scatter over the remaining Parmesan and the pine nuts. Bake for 40 mins until piping hot in the centre and crunchy and golden on top. Scatter with a few extra basil leaves before serving.