On a large baking sheet, arrange the halved tomatoes cut-side up, along with the quartered onion and unpeeled garlic cloves.
Drizzle the olive oil over the vegetables, and season with salt, black pepper, dried basil, and sugar if using. Toss gently to coat everything evenly.
Roast in the oven for about 30-35 minutes, or until the tomatoes are caramelized and the onions are soft.
Remove the baking sheet from the oven and allow it to cool slightly.
Squeeze the roasted garlic out of its skin into a blender or food processor. Add the roasted tomatoes and onions, along with any remaining juices from the baking sheet.
Pour in the vegetable broth and blend until smooth. If the soup is too thick, you can add more broth to reach your desired consistency.
Taste and adjust seasoning if necessary. You can also add a splash of heavy cream for a richer flavor, blending briefly to combine.
Heat the soup in a pot over medium heat until warm.
Serve hot, garnishing each bowl with fresh basil leaves.
Notes
Serve the soup in rustic bowls and add a drizzle of olive oil or a dollop of heavy cream before garnishing with fresh basil for an appealing look.