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Halve the squash lengthways, then scoop out the seeds. Peel, then cut the flesh into about 2.5cm pieces. Pour the stock into a pan, add the porcini, then bring to a gentle simmer.
Heat 2 tbsp of the oil in a heavy, wide pan. Add the onion, garlic, sage, thyme and squash, then gently fry for about 10 mins until the squash is almost tender, stirring occasionally, so it doesn’t stick or burn. With the heat on medium, tip the rice into the squash. Keep stirring for 3-4 mins to toast it without colouring. Pour in the wine and stir everything for 1 min.
Start to add the hot stock (leaving the porcini behind) – this process should take 18-20 mins, so put a timer on if it helps. Stir in 1½ ladles and adjust the heat so it simmers. Keep stirring and scraping down the sides. Once the first lot of stock has been absorbed, add another ladleful, continuing to stir to keep the risotto creamy. Continue adding and stirring in a ladleful of stock as each previous one is absorbed (it’s ready for more when you drag the spoon across the bottom of the pan and it leaves a clear line).
As the last of the stock goes in (keep a little back) check if the rice is ready – it should be soft with a bit of chew in the middle – and the consistency fluid. Season with pepper.
Take the pan off the heat. Add a splash of the stock to keep the risotto moist, scatter over the parsley and half the Parmesan, then spoon on the mascarpone. With the lid on, let the risotto sit for 3-4 mins to rest.
Meanwhile, heat the remaining oil in a small frying pan. Add the sage leaves, then fry for a few secs until starting to colour. Transfer to kitchen paper with a slotted spoon to drain. Spoon the risotto into bowls, then scatter over the rest of the Parmesan and the crisp sage leaves.