Remove the sausages from their skins by slitting them lengthways.
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 stock, tomato purée, and season with salt and pepper.
Bring to the boil, then simmer uncovered for about 15 minutes until thickened.
Taste and season if needed and set aside.
In a pan, add the butter, flour and milk.
Heat on low, whisking, until thickened and smooth.
Add a sprinkle of freshly grated nutmeg, season, then simmer for 2 minutes.
Preheat the oven to 190°C / 375°F / Gas mark 5.
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 everything well.
Pour half the pasta into a shallow ovenproof dish, about 2.2 litre and smooth down 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.
Let cool for 5 minutes before serving.