In a soup pot, saute leeks and onion in butter for 3 minutes. Add potatoes and chicken stock. Bring to a boil and simmer 20 minutes. Put through a sieve or into a blender to puree, and add heavy cream. Season with salt, pepper and nutmeg. Add horseradish and stain the soup if desired. Chill in an ice bath and refrigerate. Just prior to serving, ladle into bowls and garnish with salmon, dill, pickled cabbage and caviar. Nutritional info per serving: 331 cal; 10g pro, 27g carb, 22g fat (57%)