REMOVE THE ROOT END OF THE leeks, trim the dark green tops and reserve for another use. Slice the light part into 1/2-inch rounds, wash well and set aside on a plate.
Heat the oil in a pot, over medium heat, and add the leeks. Cook, stirring, for 5 minutes. Add stock, water, potatoes, nutmeg, salt and pepper. Raise heat to high, cover and bring to a boil. Reduce heat to low and simmer, uncovered, for 30 minutes.
Remove from heat and puree the soup in batches. Place pureed soup in a pot and reheat, covered, over low heat. To serve, place a generous dollop of sour cream in the bottom of each soup bowl and sprinkle with chopped chives. Transfer piping hot soup to a tureen or pitcher and pour into bowls at the table.
Refrigerate soup for up to 3 days or freeze the soup in 3-cup batches.