In a heavy 4 quart pan, melt 1/4 cup butter and onion until wilted. Add cleaned and sliced leeks, potatoes, water and salt. Bring to a boil and simmer, partially covered, for 45 minutes or until potatoes are very tender.
Remove from heat and cool if you want. Blend the cooked mixture until smooth. Return to the saucepan; add 1/4 cup butter, cream and chives. Check for seasonings. Heat, but do not boil or the cream will curdle.