Estonian Chilled Cucumber Soup

Description
Make sure it's thoroughly chilled before serving and follow it with the Estonian Cod Salad with Horseradish Sauce for a hot weather luncheon. Serve garnished with dill. Serving will be enough for 4 to 6.

Ingredients

 * 2 tablespoons unsalted butter
 * 1 small onion, finely chopped
 * 7 medium-size firm Kirby cucumbers, peeled, seeded, and grated
 * 1 cup chopped fresh dill
 * 3 cups chicken stock or canned broth
 * salt (to taste)
 * freshly ground white pepper (to taste)
 * 1 cup sour cream
 * 3 large egg yolks
 * ¼ cup dry sherry
 * grated zest of 1 lemon

Directions

 * 1) Melt the butter in a pan over medium heat.
 * 2) Add the onion and fry it until softened but not colored, about 5 minutes.
 * 3) Stir in the cucumbers and 1 cup dill, and leave it until the cucumber has softened, 5 to 7 minutes.
 * 4) Gradually stir in the stock.
 * 5) Bring to a boil, and then reduce the heat to low.
 * 6) Add salt and pepper and simmer, covered, for 5 minutes.
 * 7) Puree the soup in a food processor until smooth.
 * 8) Return to the pot and simmer for 2 minutes, then remove from the heat.
 * 9) In a bowl, whisk the sour cream with the egg yolks until blended.
 * 10) Stir in 1 cup of hot soup, then whisk the mixture back into the pot, stirring vigorously until smooth.
 * 11) Now add the lemon juice; then add more salt or freshly ground white pepper, if you wish.
 * 12) Cool and refrigerate the soup for at least 4 hours.
 * 13) Stir the soup before serving.