Vichyssoise is the finest of all cold soups! This wonderful, smooth soup can be garnished with a few finely chopped chives or a little curry powder. Heat butter in a large saucepan over medium-low heat: add leeks and onions. Sauté until soft but not browned, about 8 minutes. Add potatoes and stock; season with salt and pepper to taste.
Heat butter in a large saucepan over medium-low heat: add leeks and onions. Sauté until soft but not browned, about 8 minutes. Add potatoes and stock; season with salt and pepper to taste. Bring to a boil, reduce heat to low, and simmer gently for 30 minutes. Puree soup in a blender or food processor until very smooth.
Add the potatoes and chicken broth. Bring to a boil, then lower to a simmer. Simmer until potatoes are soft and easily pierced with a fork (about 20 minutes). Remove from heat and allow to cool for a couple of minutes. Add the mixture to a food processor or blender and puree until smooth.
Add potatoes and stock; season with salt and pepper to taste. Bring to a boil, reduce heat to low, and simmer gently for 30 minutes. Puree soup in a blender or food processor until very smooth. Set soup aside to cool; transfer into a bowl or a jug and refrigerate until chilled, at least 4 hours.