Heat the oil in a large saucepan, add the leek and cook for 3–5 minutes until soft. Stir in the garlic and fennel seeds and cook for 30 seconds. Stir in the tomato purée and cook for 1 minute more. Add the wine and bubble for 1–2 minutes, then pour over the stock and leave to simmer for 5 minutes.
Add the oregano, tomatoes, sea bream and mussels. Cover and cook for 3 minutes. Add the prawns and cook for 2 minutes more. Discard any mussels that do not open. Stir in the squid and heat through. Sprinkle over the parsley before serving with plenty of crusty bread.