Sauté the chopped onion and minced garlic in olive oil in a large pot until tender and fragrant.

Add the diced tomatoes and chicken or vegetable broth to the pot, bringing the mixture to a simmer.

Stir in the fresh basil leaves, then carefully blend the soup until smooth, using an immersion blender or transferring it to a regular blender. Return the soup to the pot, stir in heavy cream if desired, and season with salt and pepper to taste before serving.