6 tablespoons unsalted butter
8 large leeks, white part only, well rinsed and finely chopped
1 large onion, finely sliced
8 medium potatoes, peeled and thinly sliced
3 pints chicken stock
3 cups milk
salt
white pepper
3 cups cream
1/2 cup chopped chives

1. Saute leeks and onions in butter until soft and slightly colored. Add potatoes and stock and simmer for 45 minutes, or until potatoes are soft. Cool slightly, then process in a blender or food processor until pureed. Add milk and season.
2. Cool and refrigerate, overnight if possible. When ready to serve, stir in cream and sprinkle with chives.