Preheat the oven to 200°C, fan 180°C, gas 6. Put the butternut squash in a baking tray and toss with the oil. Roast for about 30 minutes, until soft.
Meanwhile, dry fry the pancetta in a non-stick pan until crisp, then set aside. In the same pan, fry the onion and chilli until soft. Add the pancetta, sage and chestnuts to the pan and heat through.
Cook the pasta according to the pack instructions. Drain, reserving 100ml of the cooking water. Stir the squash, chestnuts and reserved cooking water into the pasta. Serve, garnished with sage leaves quickly fried in the vegetable oil, if liked.