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Heat the oven to 220C/200C fan/gas 7. Toss the squash in 1 tbsp oil together with the chopped sage. Scatter into a shallow roasting tin and roast for 30 mins until brown and soft.
While the squash is roasting, prepare the risotto. Bring the stock to the boil and keep on a low simmer. In a separate pan, melt half the butter over a medium heat. Stir in the onions and cook gently for 8-10 mins until soft but not coloured, stirring occasionally. Stir the rice into the onions until completely coated in the butter, then stir continuously until the rice is shiny and the edges of the grain start to look transparent.
Pour in the wine and simmer until completely evaporated. Add the stock, a ladleful at a time, and stirring the rice over a low heat for 25-30 mins, until the rice is cooked al dente (with a slightly firm, starchy bite in the middle). The risotto should be creamy and slightly soupy.
At the same time, gently fry the whole sage leaves in a little olive oil until crisp, then set aside on kitchen paper. When the squash is cooked, mash half of it to a rough purée and leave half whole. When the risotto is just done, stir through the purée, then add the cheese and remaining butter and leave to rest for a few minutes. Serve the risotto scattered with the whole chunks of squash and the crisp sage leaves.