Bring tomato juice and vegetable stock to the boil. Mix starch with 2 tablespoons of water and stir into the juice, bring to the boil again. Heat the meatballs in the soup. Meanwhile, pluck the basil leaves from the stalks and put some aside for garnishing.
Chop remaining leaves and stir into the sour cream, season with salt. Season tomato soup with salt, pepper and paprika and garnish with sour cream and basil leaves.