Peel the shrimp, reserving the shells and heads if you're making homemade stock. Clean the mussels, removing any beards and debris. Cut the squid into rings or pieces, and chop the fish into 3/4-inch chunks.
Heat the olive oil in a large pot over medium heat. Add the chopped onion and green bell pepper and cook, stirring occasionally, until softened and translucent, about 8–10 minutes.
Add the sliced garlic and cook for another 1–2 minutes, being careful not to let it brown.
Stir in the crushed tomatoes and reduce the heat to medium-low. Simmer gently, stirring occasionally, until the sauce thickens and concentrates, about 10–15 minutes.
If using, add a splash of brandy or dry white wine and cook until the alcohol evaporates, scraping any browned bits off the bottom of the pot.
Pour in the fish stock and increase the heat to bring the mixture to a boil. Once boiling, reduce the heat to low and let it simmer for 10 minutes to meld the flavors.
Add the squid and cook for 5 minutes.
Add the fish chunks and mussels. Cover the pot and cook until the mussels open, about 3–5 minutes. Discard any mussels that do not open.
Stir in the peeled shrimp and cook just until they turn pink, about 1–2 minutes.
Season to taste with salt and freshly ground black pepper. Serve hot, garnished with chopped fresh parsley.