Rinse mussels and clams under cold water, discard any with broken shells or that don't close when tapped. Pat cod and shrimp dry with paper towels. Cut cod into 1-inch chunks and set all seafood aside refrigerated.

Base and vegetables

In a large heavy-bottomed pot or Dutch oven, heat butter and olive oil over medium heat until butter is foamy. Add diced onion, celery and carrots. Sauté, stirring occasionally, until onion is translucent and vegetables soften, about 6–8 minutes.

Add minced garlic and cook 30–45 seconds until fragrant, being careful not to brown.

Sprinkle the all-purpose flour over the vegetables and stir to coat; cook 1–2 minutes to remove raw flour taste, stirring constantly.

Deglaze and simmer

Pour in the white wine to deglaze, scraping up any browned bits from the bottom of the pot. Let the wine reduce by about half, 2–3 minutes.

Add the diced potatoes, fish stock (or clam juice), bay leaf and thyme sprigs. Increase heat to bring to a gentle simmer, then reduce heat to maintain a low simmer. Cook until potatoes are just tender, about 10–12 minutes.

Finish chowder

Once potatoes are tender, stir in the milk and heavy cream. Keep the heat on low and do not boil vigorously to prevent curdling. Simmer gently 3–4 minutes to heat through.

Season with salt, black pepper, smoked paprika and red pepper flakes (if using). Taste and adjust seasoning as needed.

Add cod chunks and shrimp to the pot. Gently simmer until the cod is opaque and flakes easily (about 4–6 minutes) and shrimp are pink and firm (about 3–4 minutes).

Add mussels and clams, pushing them into the simmering liquid. Cover the pot and cook 4–6 minutes, or until shells open. Discard any mussels or clams that do not open.

Remove and discard thyme sprigs and bay leaf. Check seasoning again and adjust salt and pepper to taste.

Ladle chowder into bowls and sprinkle with chopped fresh parsley. Serve immediately with crusty bread or oyster crackers.