Pre heat the oven to 200 degrees C / 390 F. I use a fan assisted, electric oven, please adjust according to your own oven
Place the tomatoes, onion and garlic on a large baking tray. Drizzle over the olive oil and gently toss everything to coat. Sprinkle over the sea salt and black pepper then roast in the oven for 30 - 35 minutes, until the tomatoes slightly char.
Tip the tomatoes into a large saucepan. Remove the skins from the garlic and onion then add them to the pan. Add the stock and the balsamic vinegar. Then give it a good stir.
Spoon ladles of soup into a blender then blitz, I did this in two batches. When the soup is smooth return it to a saucepan then add the basil. If it has cooled a little heat it up in the saucepan for a minute or two.
To serve, drizzle a little extra virgin olive oil over the soup and serve with crusty bread or feta toast.