Melt the butter in a large pan. Add the leeks and carrots and stir well, coating the vegetables
with the butter. Cover and cook for about 10 minutes, until the vegetables are beginning to soften
but not colour.
Pour in the stock and the orange rind and juice. Add the nutmeg and season to taste with salt
and pepper. Bring to the boil, lower the heat, cover and simmer for about 40 minutes, or until the
vegetables are tender.
Leave to cool slightly, then puree the soup in a food processor or blender until smooth.
Return the soup to the pan and add 30ml/2 tbsp of the yogurt, then taste the soup and adjust the
seasoning, if necessary. Reheat gently. Ladle the soup into warm individual bowls and put a swirl
of yogurt in the centre of each. Sprinkle the fresh sprigs of coriander over each bowl to garnish,
and serve immediately.