Preheat your oven to 200°C (390°F). Line a baking tray with parchment paper.
Cut the onions into very small cubes. Slice the fresh basil leaves finely.
Cut the red peppers in half and remove the seeds. Cut the tomatoes in half.
Place the garlic head on a small piece of aluminium foil. Drizzle with a little olive oil and sprinkle with salt. Wrap it tightly.