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Heat oven to 200C/180C fan/gas 6. Toss the squash with 1⁄2 tbsp of oil, chilli flakes and some seasoning on a large roasting tray. Roast for 15 mins, then add the chestnuts and nestle the sage and pancetta around the squash so that they touch the baking sheet. Return to the oven for 8–10 mins until the pancetta and sage are crisp. Remove from the oven and set aside. When cool, tear the pancetta and sage leaves into pieces.
Heat the remaining oil in a sauté pan over a low heat. Add the shallots and cook for 8 mins until softened but not coloured. Add the garlic and cook for 1 min. Tip in the pearl barley, toast for a minute, then pour in the stock.
Working in small amounts, add the stock to the pan and cook, stirring over a medium heat until all the liquid is absorbed by the barley – about 20 mins. When there is only a little liquid left, stir in the roasted ingredients to warm through. Season to taste. Serve with Parmesan.