Put a large sauté pan on a medium heat. And add the fennel seeds Cook for 1-2 minutes, tossing them from time to time, until they release their aroma.
Meanwhile, slit the chorizo down the side, peel and discard the casing and cut the sausage into chunks. Add the chorizo, rosemary, garlic and chilli to the pan with a little drizzle of oil and cook for 1–2 minutes, stirring.
Next add the tomatoes, wine (or stock), harissa paste (or tomato purée) and oregano. Increase the heat and leave it to bubble away for about 15 minutes so the sauce can become nice and thick and full of flavour. Give it a stir every so often to prevent it sticking.
As this cooks, put the kettle on to boil and then cook the linguine in a large pan according to the packet instructions.
Add the prawns and mussels to the sauce for the last 4–5 minutes of cooking time. If using already cooked prawns, they will take a little less time as you are simply warming them through.
Meanwhile, drain the cooked pasta well and return it to the pan, adding a good drizzle of oil and some salt and pepper, then pop the lid on to keep warm.
The prawns and mussels should now be cooked in the sauce. Place a lid on to cook the mussels. Taste the sauce, adding some salt and pepper if you think it needs it and a little sugar if it tastes too sharp. If you add sugar, stir it in well and then leave to cook for another minute.
Finally, tip the sauce into the pasta, stir well, then divide among four plates, scatter with the ripped-up parsley and serve.