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Heat the oven to 200C/180C fan/gas 6. Toss the butternut cubes on a baking tray with ½ tbsp of the oil and some seasoning. Roast for 20-25 mins or until tender.
Put the parsley, basil, cashew nuts, garlic, lemon zest and juice and chilli (if using) in a food processor, along with the remaining oil and a splash of water, then whizz until very smooth. Season to taste.
Meanwhile, cook the pasta following pack instructions. Drain, reserving a little of the cooking water, then toss with the pesto and butternut squash and enough water to loosen the sauce. Finish with a little shaved parmesan, if you like.