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Slit the sausages and remove them from their skins, then chop them into small pieces. Heat the oil in a pan, add the onion and fry for 5 minutes, until softened and lightly browned. Stir in the sausages and fry until lightly coloured. Add the carrot, then stir in the wine, stock, tomato purée, and season.
Bring to the boil, then simmer uncovered for about 15 minutes until thickened. Taste and season. Set aside.
Put the butter, flour and milk in a pan. Gently heat, whisking, until thickened and smooth. Add a sprinkle of freshly grated nutmeg, season, then simmer for 2 minutes.
Preheat the oven to 190C/Gas 5/fan 170C. Bring a large pan of salted water to the boil. Add the pasta, stir well, then cook, uncovered, for 10-12 minutes, until tender. Remove from the heat, stir in the spinach and, when just wilted, drain well.
Tip half the pasta into a shallow ovenproof dish, about 2.2 litre/4 pint, and level. Spoon over the sausage sauce, then cover with the remaining pasta. Pour the white sauce evenly over the top and sprinkle with the cheddar. Bake for 20-25 minutes until golden brown. Leave for 5 minutes before serving.