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Melt the butter in a sturdy frying pan, add the onion and gently fry until softened. Stir in the grated courgettes and rice, increase the heat and sizzle while stirring for 1-2 mins.
Add the lemon juice and a ladle of hot stock, and bubble over a medium-high heat while stirring constantly. When the liquid has just about been absorbed, add another ladleful of stock. Keep cooking like this for 20-25 mins until the rice is just tender and is creamy. Stir in the Parmesan, mascarpone and some seasoning, cover with a lid or baking sheet, and set aside for 5 mins while you cook the remaining courgettes.
Heat the remaining butter and a splash of oil in a small frying pan. Add the diced courgettes, and fry over a high heat for 2-3 mins until golden and just softened. Divide the risotto between shallow bowls or plates, then scatter with the diced courgettes and any buttery juices, the pine nuts and a few pinches of lemon zest.