Place the halved tomatoes and the head of garlic (cut side up) on a baking sheet. Drizzle with 1 tablespoon of olive oil and sprinkle with salt and pepper.
Roast in the oven for about 25-30 minutes, or until the tomatoes are slightly caramelized and the garlic is soft.
In a large pot, heat the remaining tablespoon of olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Squeeze the roasted garlic cloves out of their skins and add them to the pot with the onions. Stir for an additional minute.
Add the roasted tomatoes along with their juices to the pot, then pour in the vegetable broth. Add sugar, dried basil, and more salt and pepper to taste.
Bring the soup to a simmer and let it cook for about 15 minutes to blend the flavors.
Using an immersion blender (or transfer in batches to a blender), puree the soup until smooth.
If desired, stir in heavy cream for extra richness and simmer for an additional 5 minutes.
Serve warm, garnished with fresh basil leaves.
Notes
Optional heavy cream can be added for extra creaminess.