Heat the oil in a large saucepan and fry the onion and garlic until softened. Add the tomatoes, tomato purée, stock, orange rind and juice and basil. Bring to the boil, then reduce the heat, cover the pan and simmer gently for 20–25 minutes until the vegetables are soft.
Allow the soup to cool slightly, then purée in batches in a blender or food processor and push through a nylon sieve into the rinsed pan to remove the seeds. Season with salt, pepper and a little sugar. Return the pan to the heat and bring to the boil, then add a little extra stock or tomato juice if necessary to achieve the desired consistency.
Fold the chopped basil gently into the Greek yogurt. Pour the hot soup into warmed soup plates, spoon a little basil yogurt on each one and garnish with small basil sprigs and orange rind.