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1
To make the soup base: Melt the butter in a 6-quart stockpot over medium heat.
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2
Stir in the leeks, onion and salt.
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3
Cover and cook over low heat until the vegetables are soft but not browned, about 3 minutes.
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4
Add the chicken stock and cream.
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5
Raise the heat to medium-high and bring to a gentle boil, then simmer on low, uncovered, for 30 minutes.
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6
Puree the soup in a blender or food processor or with a hand-held immersion blender until emulsified and smooth.
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7
Return the mixture to the stockpot and set over medium-high heat.
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8
Bring to a gentle boil.
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9
Using a slotted spoon, skim off any impurities that rise to the surface.
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10
Set aside or cover and refrigerate for up to 24 hours.
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11
To finish the soup: Fill a large bowl with ice water and set aside.
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12
Bring 4 quarts water to a boil.
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13
Add the coarse sea salt and watercress.
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14
Blanch until wilted and soft, 2 to 3 minutes.
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15
Immediately drain the watercress, then plunge into the ice water to stop the cooking.
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16
Drain again; do not squeeze out all the water.
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17
Puree the watercress in a food processor, then place it in a fine-meshed sieve and press out any remaining liquid.
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18
Discard the liquid.
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19
Set the puree aside.
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20
When ready to serve, reheat the soup base over low heat.
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21
Meanwhile, using an electric mixer, whip the cream to stiff peaks.
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22
Stir in the lemon juice and season to taste with sea salt and white pepper.
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23
Just before serving, add the watercress puree to the warmed soup and stir until blended.
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24
Ladle the soup into shallow soup bowls.
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25
Place a scoop of whipped cream in the center of each bowl and top with a small spoonful of caviar.
