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Pour the stock into a saucepan and bring to the boil. Blanch the spinach and parsley in the stock for 30 seconds until wilted and bright green, then remove with a slotted spoon and leave to cool for a moment. Turn the stock down to a low heat. Squeeze out any excess water from the spinach and parsley, then tip into a food processor or mini chopper. Add 2 tbsp of the olive oil and 1 tbsp water, then blitz to a fine purée and set aside.
Heat the remaining oil in a large non-stick frying pan or skillet and fry the prosciutto until crisp. Transfer the prosciutto to a plate covered with kitchen paper and set aside. Put half the butter, the thyme leaves, leek and garlic in the frying pan and season. Scrape up any crisp bits left over from the prosciutto and fry gently for a few mins over a medium heat until the leek is softened and aromatic but not browning. Add the rice, stirring to coat in the garlic mix, and cook for 2 mins until turning translucent, then pour in the wine and cook for another 2 mins until evaporated. Add one ladleful of the warm stock and cook until absorbed, stirring continuously.
Continue this process one ladleful at a time for 20-30 mins, stirring constantly, until the rice is creamy but still retaining a little bite and without a chalky core. Remove the pan from the heat and stir through the remaining butter, the parmesan, most of the lemon zest, the lemon juice and cream (if using). Stir through the spinach purée, then season well with a little freshly grated nutmeg. Divide between two plates, top with the crisp prosciutto and sprinkle over a little more parmesan and lemon zest.