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Heat oven to 200C/180C fan/gas 6. Bring a large pan of water to the boil and cook the pasta following pack instructions. Drain, reserving 1 tbsp of the cooking water. Meanwhile, heat the oil in a large frying pan, add the chorizo and cook for 3-4 mins until crisp. Throw in the spinach, garlic and some seasoning, and cook until just wilted. Tip into a bowl with the pasta and the reserved 1 tbsp cooking water, then mix well.
Mix the ricotta, milk and lemon zest together with some seasoning. Spoon half the pasta into a gratin dish and spoon over half the ricotta mix. Top with the remaining pasta and dollop over spoonfuls of the remaining ricotta. Scatter with the Parmesan and bake for 15 mins until the cheese has melted. Grind over some black pepper before serving.