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, cooking for an additional 3 minutes until softened.
Pour in the diced tomatoes, fish stock, and white wine. Add the oregano and red pepper flakes, stirring to combine. Bring the mixture to a simmer.
Once simmering, add the fish pieces and cook for about 5 minutes.
Add the shrimp and mussels to the pot, cover, 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.