Leeks impart a delicate flavor and wonderful aroma, but if you can’t find them, substitute onions.
Melt butter in heavy skillet. Saute leeks until tender. Do not brown. Remove pan from heat. Sprinkle flour over leeks and return pan to low heat. Cook several minutes, stirring constantly, until flour is well blended. Remove from heat. Add hot water, ½ cup at a time, stirring constantly until blended. Season with salt and pepper. Heat to simmer, add potatoes, and cook, partially covered, 20 minutes or until potatoes are tender. Stir in sour cream and serve, or press mixture through food mill. Garnish with finely chopped chives.