Heat the olive oil in a large Dutch oven over medium heat.
Add the onion, celery, and carrots. Cook for 6–8 minutes until softened.
Stir in the garlic and cook for 30 seconds.
Add the tomato paste and cook for 1 minute.
Pour in the crushed tomatoes, seafood stock, and water if needed.
Stir in the paprika, Italian seasoning, thyme, bay leaf, salt, and pepper.
Bring to a gentle boil, then reduce the heat and simmer for 20 minutes.
Add the white fish and cook for 4 minutes.
Stir in the shrimp and mussels. Cover and cook for another 4–6 minutes, until the shrimp are pink, the fish flakes easily, and the mussels have opened. Discard any unopened mussels.
Remove the bay leaf and adjust the seasoning with salt, pepper, and fresh lemon juice if desired.
Garnish with chopped parsley and serve immediately with lemon wedges and warm crusty bread.