Peel the casing off the chorizo and cut into small cubes.
Heat a large pan and pan fry the chorizo and fennel seeds for a couple of minutes, stirring until the sausage starts to release its spicy oil.
Add the olive oil, onions and garlic and stir over low heat for 3-4 minutes until softened.
Stir in the tomato purée, sugar, chillies and wine.
Add the chopped tomatoes, stir well and bring to a rolling simmer.
While the sauce is bubbling away, bring a large pot of water to the boil. Add a generous pinch of salt and cook the linguine for 8 minutes until al dente. Drain well and return to the pot. Stir in the butter and toss together.
Add the pasta to the tomato sauce, using tongs to mix it until well coated. Taste and season. Stir in the spinach and serve.