Add tomatoes, tomato sauce, garlic, onion, basil, salt, pepper, heavy cream and vegetable or chicken broth to your crockpot. Cover and cook on low for 6-8 hours (although it can be done on high for 2 hours as well). Stir occasionally and scrape down the sides (if you can).
40 minutes before serving, transfer soup to a blender or food processor and puree until smooth. Return to crockpot.
Make your roux by melting your butter in a saucepan over medium heat. Once butter is melted, add the flour and whisk until it's incorporated. Slowly whisk in the heavy cream until the cream is smooth and thickened.
Add Parmesan cheese and roux to the crockpot and stir to combine.
Cook for another 20-30 minutes. Cheese should be completely melted into the soup. Stir thoroughly before serving. Garnish with additional Parmesan cheese and basil if desired.