1. In a large pot, melt the butter over medium heat. Add the chopped onion and minced garlic, and sauté until softened, about 5 minutes.
2. Sprinkle the flour over the onions and garlic, and cook for an additional 2 minutes, stirring constantly.
3. Gradually pour in the chicken or vegetable broth, stirring to combine and ensure there are no lumps from the flour.
4. Add the diced tomatoes (with their juices), sugar, and dried basil to the pot. Bring the mixture to a boil, then reduce the heat and let it simmer for 15-20 minutes, allowing the flavors to meld together.
5. If desired, use an immersion blender to puree the soup until smooth. Alternatively, you can transfer the soup in batches to a blender and blend until smooth, then return it to the pot.
6. Stir in the heavy cream, if using, and season the soup with salt and pepper to taste. Let it simmer for an additional 5 minutes.
7. Serve the tomato soup hot, garnished with fresh basil leaves if desired. Enjoy this comforting and delicious classic!

Feel free to adjust the seasoning and consistency of the soup according to your taste preferences. You can also customize it by adding other ingredients like roasted red peppers or carrots for extra flavor.