Arrange the halved tomatoes, onion quarters, and unpeeled garlic cloves on a large baking sheet.
Drizzle the olive oil over the vegetables and sprinkle with salt, black pepper, and sugar. Toss to coat evenly.
Roast in the preheated oven for 30-35 minutes, or until the tomatoes are caramelized and the onions are tender.
Remove the baking sheet from the oven and allow the vegetables to cool slightly.
In a large pot, combine the roasted vegetables, vegetable broth, and fresh basil leaves.
Bring the mixture to a simmer over medium heat and let it cook for about 15 minutes to allow the flavors to meld.
Using an immersion blender, blend the soup until smooth. Alternatively, transfer the soup in batches to a regular blender and blend until smooth, then return it to the pot.
Stir in the balsamic vinegar and adjust seasoning if needed. For a creamier texture, swirl in some heavy cream before serving.
Notes
Serve hot in bowls, garnished with fresh basil leaves and a drizzle of olive oil or cream for added elegance. Enjoy with crusty bread on the side!