Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper. Arrange the halved tomatoes cut-side up on the sheet, scattering the chopped onion and garlic around them. Drizzle with 2 tablespoons of olive oil, then season with salt and pepper.

Roast the mixture for 35–40 minutes in the preheated oven until tomatoes are charred and caramelized.

Cool the roasted vegetables for a few minutes. Transfer them to a blender or use an immersion blender in a pot to blend until smooth.

Pour the blended mixture into a large soup pot over medium heat. Add vegetable broth, red pepper flakes, and sugar. Simmer uncovered for about 10 minutes.

Stir in the fresh basil leaves and blend again until silky. If adding cream or coconut milk, do so now and let it simmer for another 5 minutes.

Adjust seasoning to taste and serve hot with croutons or bread.