Melt the butter in a 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.
Stir in ground coriander and cook for about 1 minute. Pour in the stock and add seasoning to taste. Bring to a boil, cover and simmer for about 20 minutes until the leeks and carrots are tender.
Cool slightly then puree the soup in a blender or with an immersion blender until smooth. Return to pan and add 2 tbl of the yogurt and fresh coriander. Taste and adjust the seasoning. Reheat gently but do not boil.
Ladle the soup into warmed soup into warmed soup bowls and put a spoonful of the remaining yogurt in the center or each. Serve immediately.
