Melt unsalted butter in a large pot over medium heat. Add diced onion and fennel; cook until tender and translucent, about 8 minutes.
Stir in tomato paste, minced garlic, dried oregano, and crushed red pepper flakes. Cook for 1 minute until fragrant and well combined.
Pour in dry white wine to deglaze the pot, scraping any brown bits. Then, add petite diced tomatoes, vegetable stock, clam juice, and bay leaf. Bring to a boil, then reduce heat and simmer uncovered for 15 minutes, seasoning with salt and pepper to taste.
Gently stir in the scrubbed little neck clams and mussels. Cover the pot and cook until the shellfish begin to open, about 3-4 minutes.
Incorporate cod or halibut filets, medium shrimp, and large sea scallops into the broth. Simmer until fish is opaque and shellfish are opened, about 3-4 more minutes. Discard any unopened clams or mussels.
Stir in chopped fresh parsley. Taste and adjust seasoning before serving immediately in warm bowls.
Notes
Serve with crusty bread or garlic toast to soak up the delicious broth.