On a large baking sheet, arrange the halved tomatoes cut side up, quartered red onion, and unpeeled garlic.
Drizzle with olive oil, then sprinkle the dried oregano, sugar, salt, and pepper evenly over the vegetables.
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 let it cool for a few minutes. Squeeze the garlic from its skins and discard the skins.
In a large saucepan, combine the roasted vegetables and their juices with the vegetable broth. Bring to a simmer over medium heat.
Use an immersion blender to purée the mixture until smooth, or carefully blend in batches using a regular blender. If you prefer a chunkier soup, blend only half.
Stir in the fresh basil and heavy cream (if using). Adjust seasoning with additional salt and pepper if needed. Heat through for about 5 minutes.
Serve hot, garnished with extra basil leaves and a drizzle of olive oil if desired.
Notes
Serve the soup in rustic bowls and top with a swirl of cream and fresh basil for a beautiful touch. A side of crusty bread would complement the dish perfectly!