In a large pot, melt butter over medium heat. Add chopped onion and minced garlic, sautéing until fragrant and translucent, about 3-5 minutes.

Add the chopped tomatoes to the pot and cook for 5 minutes, stirring occasionally.

Pour in the broth and bring the mixture to a simmer. Cook for 20 minutes, allowing the flavors to meld.

Use an immersion blender to purée the soup until smooth (or transfer to a blender in batches).

Stir in sugar (if using), salt, pepper, and heavy cream or milk. Heat gently, but do not boil.

Serve warm, garnished with fresh basil leaves.