Wash, peel and slice the potatoes and place in a large pot. Wash to remove any sand and dirt from the leeks and then slice them up coarsely as well and add to the pot. Add the salt and the water and bring up to a simmer and let cook for 40 minutes. Remove from the heat and let stand with the lid on for about an hour so that it has a chance to cool down a bit.
You can make this as difficult or as easy as you like, I use a hand held mixer (mixstavel) to puree the soup right in the pot to save time and cleaning up. You can also pass the solid contents of the pot through a food mill which gives a more velvety texture, or put it into the bowl of a food processor.
In any case return the contents to the pot and add the nutmeg, white pepper, cream and taste for salt. Remember that cold soups usually require a bit more salt than hot soups.
Serve chilled or hot, garnish with finely sliced chives or the tops of fresh onions or spring onions also very finely sliced. You can see more suggestions below.
If you find the soup too thick you can of course thin it with a bit of water.