Prepare the Mussels: Begin by thoroughly cleaning the mussels under cold running water. Remove any beards and discard any mussels that do not close when tapped.
Sauté the Aromatics: In a large pot, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Stir in the minced garlic and diced red bell pepper, cooking until soft, about 3 minutes more.
Build the Stew Base: Stir in the diced tomatoes, white wine, smoked paprika, and red pepper flakes if using. Bring to a simmer and let the flavors meld, about 5 minutes.
Simmer the Stew: Pour in the broth and season with salt and pepper. Bring the mixture to a low boil, then reduce the heat to maintain a gentle simmer. Cook for 10 minutes, allowing the stew to thicken slightly.
Cook the Mussels: Add the cleaned mussels to the pot, stirring gently to coat them in the sauce. Cover the pot with a lid and let the mussels steam until they open, about 5-7 minutes.
Finish with Fresh Herbs: Once the mussels have opened (discard any that haven't), sprinkle the stew with chopped fresh parsley. Give the stew a gentle stir to distribute the herbs.
Serve the Stew: Ladle the stew into bowls, ensuring each serving has an ample amount of mussels and broth. Serve immediately with crusty bread to soak up the flavorful tomato broth.