Preheat your oven to 400°F (200°C).
Arrange the halved tomatoes, onion, and garlic on a baking sheet.
Drizzle with olive oil, and season with salt, pepper, basil, and oregano.
Roast in the oven for 25-30 minutes, until the tomatoes are caramelized.
Remove from the oven and let cool slightly.
Squeeze the garlic from its skins and add to a blender with the roasted vegetables.
Pour in the vegetable broth and balsamic vinegar.
Blend until smooth, adjusting the consistency with more broth if necessary.
Taste and adjust seasoning as needed.
Heat the soup on the stove before serving, garnished with fresh basil.