My eyes light up whenever I see watercress soup, with its brilliant green color and slightly acidic bite that is delicious hot or chilled. However, watercress must be blanched and then thoroughly simmered in broth before any milk is added, or the acid in the watercress will cause the milk to curdle. This piquant green can be difficult to cultivate, prospering in currents of clean river water. It flourishes around Tours, where this soup originated. For Cream of Spinach Soup, substitute
Wash the watercress, discard the stems, and reserve the sprigs, setting aside 12 leaves for decoration. Bring a large pot of water to a boil, add the watercress, and blanch for 1 minute. Drain, rinse under cold running water, then squeeze the watercress dry with your hands.
Melt the butter in a soup pot over low heat. Add the blanched watercress and cook, stirring, until the moisture has evaporated, 2 to 3 minutes. Add the potato and broth, season with salt and pepper, and bring to a boil. Cover and simmer, stirring occasionally, until the potato is tender, 15 to 20 minutes.
Purée the soup in a food processor or in the pot using an immersion blender. It can be prepared up to this point 2 days ahead and stored in the refrigerator.
To finish, stir the milk into the vegetable purée and reheat the soup almost to boiling. Stir in the crème fraîche and nutmeg and bring it just back to a boil. If the soup is too thick, thin it with a little milk. Taste and adjust the seasoning with salt, pepper, and nutmeg. This soup should be light and refreshing. Serve it hot, or let it cool, chill well, and serve in chilled bowls. Decorate each serving with the reserved watercress leaves.
© 2007 All rights reserved. Published by Chronicle Books.