Discard any shellfish that cracked or open shells.
Drizzle olive oil in large pot. Add onions and celery. Saute onions over medium-high heat until soft, about five minutes. Add garlic and cook for about one minute. Add Italian seasoning and thyme. Stir until aromatic, about two minutes.
Stir in tomatoes and fish stock. Bring to boil and simmer for about 15 minutes.
While soup is simmering, combine whole clams and mussels in large saucepan. Add white wine. Cook over medium-high heat until shells open, about five minutes. Discard any that remain closed. Reserve broth.
Add fillet, cooked shellfish and wine broth to soup. Add cooking sherry and cayenne pepper. Stir ingredients and bring to boil. Add salt and pepper to taste.