Heat olive oil in a large, heavy-bottomed pot over medium heat. Add the fennel and onion, cooking for 8-10 minutes until softened and starting to caramelize. Add garlic and cook for 1 more minute.
Pour in the white wine and let it simmer for 3-4 minutes, scraping up any browned bits from the bottom of the pot.
Add the crushed tomatoes, seafood stock, red pepper flakes, bay leaf, and oregano. Bring to a boil, then reduce heat and simmer for 20 minutes to let the flavors meld together.
Season the broth with salt and pepper to taste. Add the fish pieces first and cook for 3 minutes, then add the shrimp and cook for another 2 minutes.
Add the mussels, cover the pot, and cook for 4-5 minutes until the mussels have opened. Discard any mussels that don't open.
Remove the bay leaf and stir in fresh parsley. Serve immediately with crusty bread, or refrigerate and gently reheat the next day for even richer flavor. When reheating, be careful not to overcook the seafood—just warm it through gently!