In a large pot, heat the olive oil over medium heat. Add the onion and garlic, sautéing until the onion becomes translucent, about 3-4 minutes.
Stir in the bell pepper, carrot, and celery. Cook for another 5 minutes until the vegetables soften.
Add the dried thyme, smoked paprika, and red pepper flakes (if using). Stir well to combine and cook for 1 minute until fragrant.
Pour in the diced tomatoes and seafood stock. Bring the mixture to a simmer and let it cook for about 10 minutes.
Gently add the chunks of white fish to the pot, followed by the shrimp, mussels, and clams. Cover the pot and cook for an additional 5-7 minutes, or until the seafood is cooked through and the mussels and clams have opened.
Season with salt and pepper to taste. Remove from heat and let it sit for a couple of minutes.
Serve hot, garnished with fresh parsley and lemon wedges on the side.