Preheat your oven to 400°F (200°C).

On a large baking sheet, arrange the halved tomatoes, chopped onion, and garlic cloves. Drizzle with olive oil and sprinkle with salt, black pepper, oregano, and red pepper flakes if using. Toss to coat evenly.

Roast in the preheated oven for 25-30 minutes, or until the tomatoes are caramelized and the onions are tender.

Remove the baking sheet from the oven and let it cool slightly. Transfer the roasted vegetables to a large pot.

Add the vegetable broth to the pot and bring to a simmer over medium heat. Cook for about 10 minutes.

Stir in the fresh basil leaves and balsamic vinegar. Using an immersion blender, blend the soup until smooth. Alternatively, you can carefully transfer the soup in batches to a blender.

If desired, stir in the heavy cream for a richer flavor and creamier texture. Heat through for an additional 2-3 minutes.

Taste and adjust seasoning if necessary. Serve hot, garnished with additional basil if desired.