In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 5 minutes.
Stir in the minced garlic and diced bell pepper. Cook for another 3 minutes until softened.
Add the chopped tomatoes, thyme, and smoked paprika. Stir and cook for an additional 2-3 minutes to combine the flavors.
Pour in the fish or seafood stock and white wine (if using). Bring the mixture to a simmer and let it cook for 10 minutes, allowing the flavors to meld.
Add the shrimp and white fish to the pot. Cook for about 5-7 minutes until the fish is cooked through and the shrimp are pink.
Finally, stir in the cleaned mussels. Cover the pot and let cook for another 5 minutes, or until the mussels have opened. Discard any mussels that do not open.
Season the stew with salt and pepper to taste.
Serve hot, garnished with chopped fresh parsley and lemon wedges on the side.