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1
Heat the olive oil in a very large saucepan.
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2
Add the leeks, celery and potato and cook over moderately low heat, stirring often, until the vegetables are slightly softened, about 10 minutes.
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3
Add the chopped lettuce by large handfuls, stirring each batch until wilted before adding the next.
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4
Add 2 cups of the vegetable stock and season with salt and pepper.
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5
Reduce the heat to moderately low.
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6
Cover and simmer the soup, stirring often, until the vegetables are very soft, about 20 minutes.
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7
Add the remaining 2 cups of stock, cover and cook for 20 minutes longer.
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8
Working in batches, carefully puree the hot soup in a blender.
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9
Pour the soup into a clean saucepan and season with salt and pepper.
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10
In a medium bowl, prepare an ice water bath.
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11
Set aside 8 small watercress sprigs.
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12
In a medium saucepan of boiling salted water, blanch the remaining watercress and the basil leaves for 5 seconds.
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13
Drain and plunge into the ice water bath to cool.
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14
Drain again and squeeze out any excess water, then coarsely chop.
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15
Transfer the chopped watercress and basil to a blender.
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16
Add the water, chives, extra-virgin olive oil and marjoram and blend until emulsified.
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17
Transfer the puree to a bowl and season with salt and pepper.
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18
Gently reheat the soup and ladle into warmed shallow bowls.
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19
Drizzle some Tuscan olive oil and a rounded tablespoon of the watercress puree over the soup.
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20
Garnish with the reserved watercress sprigs and serve.
