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1
Place a large, heavy soup pot or Dutch oven over medium heat and add 2 tablespoons of the oil.
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2
Add the leeks and cook, stirring frequently, until tender, about 5 minutes (do not allow to brown).
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3
Add the potatoes and cook, stirring occasionally, until slightly softened, about 5 minutes more.
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4
Add half of the watercress and cook, stirring, just until wilted, about 3 minutes more.
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5
Add the broth, salt, and pepper to taste.
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6
Bring to a boil, then reduce the heat to low and simmer gently, partially covered, until the vegetables are very tender, about 25 minutes.
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7
Remove the soup from the heat and let cool for 5 to 10 minutes.
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8
In batches, process the soup in a blender or food processor until very smooth.
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9
(Hold the top of the blender firmly with a folded towel to prevent an explosion of hot soup.)
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10
Transfer the soup to a large bowl.
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11
Stir in the half-and-half and 1/2 tablespoon of the lemon juice.
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12
Cool to room temperature, then cover and refrigerate for at least 5 hours ( or overnight).
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13
Refrigerate the remaining bunch of watercress separately.
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14
Chill wide, shallow bowls.
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15
Taste the soup for seasoning and adjust with salt, pepper, and lemon juice as necessary.
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16
Just before serving, toss the remaining watercress with the remaining tablespoon of lemon juice and remaining tablespoon of olive oil.
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17
Season to taste with salt and pepper.
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18
Ladle into the chilled bowls and place a mound of the watercress salad on top.
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19
Scatter with a pinch each of lemon zest and chives, and serve immediately.
