Prepare your ingredients and get your equipment ready.
Spread the chopped tomatoes, onion, pepper, celery, chilli and garlic out on a baking tray. Pour over the olive oil and balsamic vinegar and sprinkle the salt and pepper. Give everything a good toss before putting in the oven for 35-40 minutes, turning once halfway through.
When the vegetables are nicely roasted, with a little char here and there, remove from the oven and transfer them into a large saucepan. Carefully remove the two halves of garlic. Scoop out the cloves from one half and drop them back into the saucepan. Store the other half away for another recipe!
Add 600ml vegetable stock to your saucepan. Bring to the boil and then turn off the heat.
Add the fresh basil (stalks and all) into the saucepan and stir it into the soup. Now, blend with an immersion blender or whizz in a blender until smooth and creamy. If the soup is too thick, add the remaining 200ml vegetable stock.
Serve in bowls or cups and top with your favourite toppings.