Pre-heat the oven to 180°C (fan)/200°C/gas mark 6.
Place the tomatoes, onion, garlic and olive oil in a large roasting tin. Stir and roast for 20 minutes. Stir and return to the oven for a further 20 minutes. Add the basil leaves for the last five minutes.
Transfer to a large saucepan and add the vegetable stock, balsamic vinegar and season with salt and pepper to taste.
Bring to the boil, then allow to simmer for 10 minutes before pulsing with a hand held blender* to the desired consistency. Serve immediately with bread.
*If you are using a jug blender allow to cool first.