On a large baking sheet, toss the halved tomatoes, quartered onion, and whole garlic cloves with olive oil, salt, and pepper.
Roast for 30-40 minutes, or until the tomatoes are softened and slightly caramelized, and the onion is tender.
Transfer the roasted vegetables to a blender or food processor.
Add the vegetable broth and blend until smooth.
Pour the blended soup into a pot and heat over medium heat. Stir in the heavy cream and warm through, but do not boil.
Taste and adjust seasoning with salt and pepper if needed.
Serve hot, garnished with fresh basil if desired.
Notes
For a dairy-free version, omit the heavy cream and add a splash of coconut milk or cashew cream. You can also add a pinch of sugar to balance the acidity of the tomatoes if needed.