Finely chop the onion finely and sweet potato into chunks.
Melt the oil in a large pot and sauté the onion, a pinch of salt and the sweet potato for 5 minutes until the onion browns. Crush the garlic and add it in stirring for 30 seconds, then add the paprika and cayenne, stir again for a few seconds then pour over the tomatoes, stock and a big pinch of salt. Let this simmer for 15 minutes.
While this is simmering, prep the fish. Make sure the mussels are scrubbed and beards removed (you can ask your fish monger to do this), chop the cod up into chunks and leave to the side with the prawns.
Once the sauce has simmered long enough, throw the seafood mix into the pot and stir well so all the fish is covered in the sauce, then pop the lid on and allow this to cook for 5 minutes until the mussels are open and the prawns are pink.
Throw over the juice of 1 lemon and the parsley, stir well and serve hot.