Heat 2 tablespoons olive oil in a large heavy pot or Dutch oven over medium heat. Add the chopped onion and sliced fennel and cook, stirring occasionally, until softened and beginning to color, about 6–8 minutes. Add the minced garlic and cook until fragrant, about 30–45 seconds.

Deglaze and build base

Increase heat to medium-high and add the white wine to deglaze the pot, scraping up any browned bits. Let the wine reduce by about half, 2–3 minutes. Stir in the tomato paste and cook 1 minute. Add the crushed tomatoes, fish stock, dried oregano, red pepper flakes, lemon zest, salt and pepper. Bring the mixture to a gentle simmer.

Simmer

Lower the heat to maintain a gentle simmer and cook the broth for 15 minutes to develop flavor. Taste and adjust seasoning (salt, pepper, red pepper flakes) as needed. Stir in half of the chopped parsley.

Seafood

Prepare seafood

While the broth simmers, rinse and pat dry the fish pieces and shrimp. Make sure mussels and clams are scrubbed and have been checked for any open shells (discard any that remain open).

Add firm fish

Gently slide the firm white fish pieces into the simmering broth. Cook for 4–5 minutes, just until the edges begin to flake but the pieces still hold shape.

Add shellfish and calamari

Add the clams and mussels to the pot, nestling them into the broth. Scatter the calamari rings and tentacles over the top. Cover the pot and steam for 5–6 minutes, or until the mussels and clams have opened (discard any that do not open).

Finish with shrimp

Uncover, stir gently, then add the shrimp and continue to simmer uncovered for 2–3 minutes until shrimp are pink and opaque and the calamari is tender (avoid overcooking).

Finishing & Serving

Brighten and rest

Remove the pot from the heat. Stir in the remaining parsley, the juice of the lemon, and 1 tablespoon extra-virgin olive oil. Taste and adjust seasoning one last time.

Serve

Ladle the stew into warmed bowls, making sure each portion gets a mix of fish, shrimp, mussels and clams. Drizzle a little extra-virgin olive oil over each bowl, scatter torn basil leaves and, if desired, a light sprinkle of grated Pecorino or Parmesan. Serve immediately with thick slices of crusty bread for dipping.

Make-ahead and storage

If preparing ahead, cook the broth base (aromatics through simmer step) up to 2 days in advance and refrigerate. Add seafood and finish just before serving to prevent overcooking. Leftovers can be refrigerated up to 2 days; reheat gently on the stove and add a splash of stock if reduced.