Heat the oven to 200°C (400°F) and line a large baking tray.
Spread the halved tomatoes, red peppers (cut side down), onion, and garlic on the tray. Drizzle with the olive oil and season with salt and pepper.
Roast for 30 to 35 minutes, until the peppers are blistered and the tomatoes are soft and caramelized at the edges.
Let cool for a few minutes, then peel the pepper skins if you want a smoother soup and squeeze the garlic out of its skins.
Tip everything into a pot, add the vegetable broth and basil, and simmer for 5 to 10 minutes.
Blend until smooth with an immersion blender, then stir in the plant cream if using.
Taste and adjust the salt and pepper, add chili flakes if you like, and serve warm.
Notes
Roast the tomatoes first for a deeper flavor. Use about 800 g canned tomatoes out of season. Stir in plant cream for a creamy version. Freezes up to 3 months.