Heat a large saucepan or medium pot over medium heat. Add olive oil and butter. When butter melts and foams, add diced onion, celery and carrot. Sauté, stirring occasionally, until vegetables are softened and translucent, about 6–8 minutes.

Add garlic and tomato paste

Add minced garlic and cook 30–45 seconds until fragrant. Stir in tomato paste and cook 1 minute to remove raw flavor, stirring constantly.

Deglaze and simmer

Pour in the white wine to deglaze the pan, scraping any browned bits from the bottom. Let the wine reduce by half, about 2–3 minutes. Add the fish stock (or clam juice), water, chopped tomato, bay leaf, thyme, crushed red pepper, salt and black pepper. Bring to a simmer, reduce heat to low, and simmer gently for 12–15 minutes to develop flavor.

Seafood and Add-ins

Prepare fennel and tomatoes

If using fennel, add thinly sliced fennel to the simmering broth now and cook 3–4 minutes until slightly softened. Taste the broth and adjust seasoning with a pinch more salt if needed.

Add firm fish

Add the firm white fish pieces to the simmering broth. Simmer gently for 3–4 minutes, until the fish is opaque and nearly cooked through.

Add shellfish

Add scallops and shrimp to the pot. If using mussels, add them now as well. Cover and simmer over medium-low heat for 4–6 minutes, until shrimp are pink and curled, scallops are opaque and firm, and mussels have opened (discard any that remain closed). Do not overcook—seafood should be just cooked through.

Finish seafood

Stir in chopped parsley and lemon juice. Taste and adjust seasoning for salt, pepper and acidity. If you want a richer soup, stir in the heavy cream or crème fraîche now and heat gently (do not boil) until warmed through.

Finishing

Remove and discard the bay leaf and thyme sprig. Ladle the soup into warmed bowls, garnish with extra parsley and serve with lemon wedges on the side for squeezing. Serve immediately.

Estimated storage: Refrigerate leftovers in an airtight container up to 2 days; reheat gently on the stovetop to avoid overcooking seafood (better to store broth and seafood separately if you anticipate reheating).