Melt butter in a large stockpot or Dutch oven over medium high heat. Add onion and fennel. Cook, stirring occasionally, until tender, about 8 minutes.

Stir in tomato paste, garlic, oregano and red pepper flakes until fragrant, about 1 minute.

Stir in wine, diced tomatoes, vegetable stock, clam juice and bay leaf. Bring to a boil; reduce heat and simmer until flavors have blended, about 15 minutes; season with salt and pepper, to taste.

Stir in clams and mussels. Reduce heat to low; cover, with a tight-fitting lid, and cook until the clams and mussels are just beginning to open, about 3-4 minutes.

Stir in cod or halibut, shrimp and scallops. Reduce heat and simmer until cod or halibut, shrimp and scallops are just cooked through and clams and mussels have opened completely, about 3-4 minutes. Discard any unopened clams.

Stir in parsley; season with salt and pepper, to taste.

Serve immediately.