In a large heavy saucepan, bring the leeks, potatoes, water, and salt to the boil over high heat. Cover partially, reduce heat, and simmer for 20 to 30 minutes until the vegetables are tender. Correct the seasoning, to taste, and puree with a handheld immersion blender, or in batches in a food processor.
Whisk in the cream and reheat before serving. Top each serving with a dollop of creme fraiche and sprinkling of fresh chives.
Recipe courtesy Julia Child, Julia's Kitchen Wisdom, Alfred A. Knopf, 2000