In a saucepan, heat olive oil over medium heat. Add chopped onion and sauté until translucent, about 5 minutes.

Add minced garlic and cook for an additional minute until fragrant.

Stir in the diced tomatoes (with juice) and vegetable broth. Bring to a simmer and cook for 20 minutes.

Remove the saucepan from heat and add the fresh basil. Use an immersion blender to puree the soup until smooth. Alternatively, transfer to a blender in batches.

Return the blended soup to the saucepan. Stir in the heavy cream, salt, pepper, and sugar. Heat gently until warmed through.

Ladle the soup into bowls and garnish with additional basil if desired. Serve hot.