In a large saucepan, cook the leeks and onion in the butter until soft, but not brown.
While the leeks and onions cook, peel and slice the potatoes to about 1/4 inch thick. Boil the slices for 10 minutes, strain, and add them to the leek and onion mixture.
Add the vegetable stock to the saucepan. Cover and simmer for 30 to 40 minutes.
Remove from heat and cool, then sieve or puree the soup until smooth.
Return the soup to the heat and stir in the milk, cream, nutmeg, salt, and pepper. Bring slowly to a boil, and then remove from the heat. Cool for about 10 minutes and then blend or sieve a second time.
Serve immediately with a dollop of sour cream and some chopped chives.