01 - Melt the butter in a large pot over medium heat. Add the leeks and onion, cooking gently for 6-8 minutes, stirring occasionally, until softened but not browned.
02 - Add the diced potatoes and stir to coat with the butter and vegetables.
03 - Pour in the stock, bring to a boil, then reduce heat to a simmer. Cover and cook for 20 minutes, or until the potatoes are very tender.
04 - Remove from the heat. Use an immersion blender to purée the soup until smooth, or transfer in batches to a blender and blend carefully.
05 - Stir in the cream and milk. Return to low heat and warm gently; do not boil. Season with salt and pepper to taste.
06 - Ladle into bowls, garnish with chopped chives, and serve with crusty bread if desired.