Combine diced tomatoes, onion, and garlic in the slow cooker.
Pour in vegetable broth and add dried herbs.
Season with salt and pepper, then stir well.
Cover and cook on low for 6-8 hours or high for 3-4 hours.
Use an immersion blender to puree the soup until smooth.
Stir in heavy cream for a creamy texture.
Taste and adjust seasoning if necessary.
Serve hot, garnished with fresh basil leaves.