Place the chopped tomatoes in a medium-sized pot and add the courgette, red onion, parsley, mozzarella, chorizo, salt and pepper. Add the stock and bring to a boil. Place the spaghetti upright in the pot and bring to a boil over a medium heat – press the spaghetti down as it softens. Allow the dish to cook, still over a medium heat, covered, for about 10 min., or until the spaghetti is al dente. Stir the pot occasionally. Sprinkle the rest of the mozzarella over the pasta dish, garnish with parsley and serve immediately straight from the pot.