In a medium saucepan, heat 1 tablespoon olive oil and 1 teaspoon butter over medium-low heat. Add the finely minced shallot and the minced garlic. Cook gently, stirring, until softened and translucent but not browned, about 3–4 minutes.

Add the white wine (if using) and simmer briskly for 1 minute to burn off alcohol and deglaze the pan, scraping any browned bits from the bottom.

Pour in 3 cups fish or seafood stock and 1 cup water. Add the thyme sprigs and bay leaf. Bring to a gentle simmer.

Vegetables & base

Add the diced tomato (or canned tomatoes), thinly sliced fennel (if using), diced potato (if using), and the 1/2 diced shallot or small onion to the simmering broth.

Season with 1/2 teaspoon of the salt, 1/8 teaspoon black pepper, and the red pepper flakes if using. Simmer gently until vegetables are tender—about 8–10 minutes for potatoes and fennel. Keep uncovered or partially covered so flavors concentrate.

Once vegetables are nearly tender, remove and discard the thyme sprigs and bay leaf.

Seafood

Taste the broth and adjust seasoning with the remaining salt and pepper as needed, keeping in mind the seafood will add brininess.

Add the firm white fish pieces first to the simmering broth, because they require slightly longer cooking (about 2–3 minutes depending on size). Gently simmer; do not boil vigorously.

After 1 minute, add the shrimp and scallops. If using mussels or clams, add them at the same time as the shrimp so they have 4–6 minutes to open—cover the pot and steam until shells open (discard any that remain closed). Cook until shrimp are pink and opaque and scallops are firm, about 2–3 minutes more.

If using very small bay scallops, add them with the shrimp and cook until just opaque, about 1–2 minutes.

Finishing & serve

Turn off the heat. Stir in the lemon zest and 1 tablespoon lemon juice to brighten the broth. Drizzle 1/2 teaspoon olive oil over the top and add 1 tablespoon chopped parsley.

Ladle the soup into warmed shallow bowls so it feels small and delicate. Offer additional lemon wedges at the table and crusty bread if desired.

Serve immediately. Leftovers can be refrigerated in an airtight container for up to 48 hours; reheat gently without boiling to avoid overcooking the seafood.