In a large pot, heat the olive oil over medium heat. Add the chopped onion and minced garlic and sauté until they become translucent and fragrant.
Add the diced carrot, diced celery, salt, pepper and dried oregano to the pot. Cook for a few minutes until the vegetables begin to soften.
Pour in the white wine and let it simmer for a couple of minutes to cook off the alcohol.
Stir in the canned diced tomatoes, and mix with the vegetables. Add the fish or vegetable broth to the pot and bring it to a simmer. Let it cook for about 10 minutes to allow the flavors to meld together.
Add the mussel meat to the pot and let it simmer for about 5-7 minutes until the mussel meat is cooked through and tender. Remove the pot from the heat.
Ladle the soup into bowls, making sure to distribute the mussel meat and vegetables evenly. Garnish with freshly chopped parsley and Parmesan and enjoy!