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1
Heat a large saucepan with olive oil over medium-low heat.
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2
Add the leeks and the watercress stems only and saute until wilted, about 10 minutes.
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3
Add the chicken stock and potatoes and season well with salt and pepper.
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4
Bring the liquid to a boil, then reduce the heat and simmer until the potatoes are tender, 25 to 30 minutes.
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5
Meanwhile, prepare an ice water bath by partially filling your largest mixing bowl with ice cubes and water.
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6
In batches to avoid overfilling and splattering, ladle the soup into a blender or food processor, adding some of the creme fraiche and watercress leaves with each batch and reserving about 2 dozen whole leaves for a garnish.
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7
Puree each batch and transfer to a fine-meshed strainer held over a large mixing bowl that fits inside the ice water bath, using a rubber spatula to press the puree through the strainer.
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8
Taste the puree, adjust the seasonings, if necessary, with more salt and pepper, and stir in the lemon juice.
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9
Place the bowl of soup inside the ice water bath and chill it until cool, stirring occasionally, about 1 hour.
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10
Remove the bowl of soup from the ice water bath, dry its underside, cover with plastic wrap, and refrigerate until thoroughly chilled.
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11
Before serving, taste the soup.
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12
If it is too thick, thin it with a little cold milk, chicken stock, or water, to a thick, creamy consistency.
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13
Adjust the seasonings, if necessary.
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14
Ladle into chilled bowls and decoratively garnish each serving with a few of the reserved watercress leaves.
