Heat olive oil in large soup pot over medium meat.
Add onion, carrot, celery and garlic. Sauté 8-10 minutes until vegetables are softened and slightly browned.
Add passata, thyme, salt and pepper. Bring to a boil, reduce heat and simmer until thickened, about 15 minutes.
Add clam juice and water. Bring to a boil.
Add mussels and clams then cover pot with a tight fitting lid. Shake the pot to distribute shellfish evenly in the broth. Reduce heat and cook for 2-3 minutes until the shells start to open.
Remove lid, add red snapper, shrimp and parsley. Gently stir to combine.
Turn heat up to bring soup back to a boil quickly. Put lid on, reduce heat and simmer 2-3 minutes until shrimp are pink and mussels and clams are all open. Discard any that did not open.
Taste broth and season with salt and pepper.
Divide fish evenly among 6 bowls and cover with broth.
Garnish with garlic croutons if desired.