Cook the pasta according to the packet instructions. Add the spinach to the cooking water for just the last 60 seconds.
Meanwhile, finely chop the onion and dice the chorizo & pepper. Heat a frying pan and add the chopped chorizo, red pepper and onion. Gently cook for about 10 minutes, until the chorizo has released its fat and the onion has softened. Add the tomato purée and stir for a minute or so.
Add a ladleful of the cooking water of the pasta & spinach to the chorizo mixture. Let it bubble and reduce to a thick sauce.
Drain the pasta & spinach and add to the chorizo sauce. Combine well, season to taste and serve.