Melt butter in a large stock pot over medium heat.
Add onions, carrot, garlic, oregano, salt and pepper and cook until vegetables are tender, about 5 minutes. Stir in flour and cook for 1-2 more minutes.
Whisk in broth and cook, stirring constantly until mixture thickens. Stir in tomatoes and heavy cream.
Add Parmesan and heat through until cheese is melted. Season with additional salt and pepper, to taste. Stir in basil just before serving.