Clean the mussels by removing any beards and grit. Cut the squid into rings or pieces and chop the fish into 2–3 cm chunks.
Heat the olive oil in a large saucepan over medium heat. Add the chopped onion and green pepper and cook for 8–10 minutes, until softened and translucent.
Add the sliced garlic and cook for another 2 minutes, taking care not to let it brown.
Stir in the chopped tomatoes and simmer over medium-low heat for 10–15 minutes, stirring occasionally, until the sauce has reduced and thickened slightly.
Pour in the cognac or white wine, if using, and cook until the alcohol has evaporated, scraping the bottom of the pan to deglaze.
Pour in the fish stock and bring to the boil. Reduce the heat and simmer gently for 10 minutes to allow the flavours to develop.
Add the squid and cook for 5 minutes.
Add the fish and mussels. Cover and cook for 3–5 minutes, or until the mussels have opened, discarding any that remain closed.
Stir in the prawns and cook for 1–2 minutes, until they turn pink.
Season with salt and freshly ground black pepper to taste. Serve hot, sprinkled with chopped parsley.