1. Melt butter in a large saucepan and cook onion, potato or sweet potato and carrots for 5 minutes.

2. Stir in stock and bring to the boil. Then reduce heat and simmer for 30 minutes. Remove pan from heat and set aside to cool slightly.

3. Transfer soup mixture, in batches, to a food processor or blender and process until smooth. To the last batch of soup mixture add the sour cream. Return soup to a clean saucepan and cook over a low heat, stirring constantly, until soup is hot. Do not allow the soup to boil or it will curdle. Stir in chopped dill and ladle soup into warmed bowls. Garnish with extra dill sprigs. Serve immediately with crusty bread and a dollop of cream or a little grated cheese if liked.