In a Dutch oven or large pot, heat 1/4 cup oil over medium-high heat until shimmering. Add shrimp shells and cook, stirring frequently, until bright pink and beginning to brown in places. Using a slotted spoon or spider skimmer, remove shells; discard shells.

2. Add onion, fennel, and 1/2 teaspoon salt to pot and cook, stirring occasionally, until softened and beginning to brown, 7 to 9 minutes. Stir in garlic, oregano, and red pepper flakes and cook until fragrant, about 1 minute. Stir in 3/4 cup white wine and saffron and cook until wine is reduced by about half, about 5 minutes.

Stir in clam juice and tomato passata and simmer, stirring occasionally, until reduced by about half, 15 to 20 minutes.

Reduce heat to low, nestle in squid, cover, and gently simmer for 15 minutes. Nestle in cod, cover, and continue to gently simmer until just cooked through, 5 to 8 minutes. Add shrimp, submerging them in the liquid, and continue to cook, covered, until all seafood is cooked through, 2 to 4 minutes. Remove from heat.

While the shrimp are cooking, in a 12-inch skillet, bring clams, 1/4 cup wine, and 1 tablespoon olive oil to boil, covered, over high heat. Steam until clams just open, 5 to 8 minutes. Transfer opened clams to pot with seafood.

Once all clams have been transferred to pot, add mussels to remaining liquid in the skillet, cover, and cook over high heat until mussels have opened, 2 to 4 minutes. Transfer opened mussles with about 1/2 cup remaining cooking broth to pot with seafood.

Return seafood stew to a simmer to warm through. Stir parsley into stew and season with salt and pepper to taste. Divide seafood among serving bowls. Ladle broth over seafood, making sure each portion contains a balance of everything. Drizzle each portion with olive oil, garnish with fennel fronds, if desired, and serve.