In a large pot, heat the olive oil over medium heat. Add the diced onion, garlic, carrot, and celery. Sauté for about 5-7 minutes, or until the vegetables are softened.
Stir in the diced tomatoes, white wine, fish stock, oregano, and red pepper flakes. Bring the mixture to a simmer and let it cook for about 10 minutes to allow the flavors to meld.
Add the chunks of white fish to the pot and simmer for an additional 5 minutes.
Next, add the shrimp and mussels. Cover the pot and cook for another 5-7 minutes, or until the mussels have opened and the shrimp are pink and opaque. Discard any mussels that do not open.
Season the stew with salt and pepper to taste. Serve hot, garnished with fresh parsley and accompanied by crusty bread for dipping.