This version calls for celery and a splash of white wine.
Melt butter in soup kettle. Saute leeks, onions, and celery until tender. Add stock and potatoes, simmer 15 minutes or until potatoes are tender. Press through food mill. Return to kettle, add cream and wine. Stir over low heat until smooth. Season with salt and pepper. Cool, chill, and serve cold, garnished with chopped chives.