Heat olive oil in a large Dutch oven over medium heat. Add onion and fennel and cook 6-7 minutes until softened. Stir in garlic and red pepper flakes and cook 1 minute until fragrant.
Pour in the white wine and simmer 2-3 minutes, scraping up any browned bits, until slightly reduced.
Add the crushed tomatoes, seafood stock, bay leaf, and oregano. Bring to a simmer and cook uncovered for 20 minutes to let the flavors meld.
Nestle the fish fillets and mussels into the broth first, cover, and simmer 4 minutes. Add the shrimp and calamari, cover again, and cook 3-4 more minutes, until the mussels have opened, the shrimp are pink, and the fish flakes easily. Discard any mussels that don't open.
Discard the bay leaf and stir in half the parsley.
Ladle into wide bowls, top with remaining parsley, and serve with lemon wedges.
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