Heat the oil in a heavy-based pan with a lid. Add the onion and cook for 5 mins, or until softened. Stir in the garlic and paprika, cook for 2 mins, then pour in the tomatoes and stock; season. Bring to the boil, then reduce to a simmer for 10 mins.
Add the fish chunks and continue cooking for 2 mins. Add the prawns and mussels, then cover and cook for 3 mins. Discard any mussels that haven’t opened, then scatter over the parsley.
Spoon into shallow bowls and serve with crusty bread and butter, if you like.