1. Melt butter and oil in the saucepan. Add the leeks, onion and bacon and cook for 5 minutes until the onion is soft. Stir in the potatoes and cook for 15 minutes, stirring frequently.
2. Add the chicken stock and salt and pepper to taste and simmer for 30 minutes or until the potatoes are very tender.
3. Leave the mixture to cool for 30 minutes and then place the mixture into a blender or food processor and blend until smooth. Return the soup to the saucepan and reheat over medium heat until warm. Pour into bowls and garnish with chives.