Ingredients

Makes about 8 cups

　　1 1/4 pounds sweet onions such as Vidalia, chopped (about 4 cups)

　　1/2 stick (1/4 cup) unsalted butter

　　1/4 cup all-purpose flour

　　4 cups chicken broth

　　2 cups water

　　3 cups loosely packed fresh flat-leafed parsley leaves, washed well and spun dry

　　1 cup chopped fresh chives (about 3 bunches)

　　6 tablespoons loosely packed fresh tarragon leaves

　　3 tablespoons fresh thyme leaves (preferably lemon thyme)

　　3/4 cup sour cream

　　1 1/2 tablespoons fresh lemon juice, or to taste

　　Garnish: sour cream, finely chopped fresh chives, and fresh thyme sprigs

　　

Step 1

In a 4- to 5-quart saucepan cook onions in butter with salt and pepper to taste over moderately low heat, stirring, until softened. Add flour and cook, stirring, 3 minutes. Stir in broth and water and simmer, stirring, 2 minutes, or until slightly thickened. Add herbs and simmer 1 minute.

　　

Step 2

Remove pan from heat and let mixture cool slightly. In a blender puree mixture in small batches until very smooth, forcing as pureed through a fine sieve into a bowl. Whisk in sour cream and salt and pepper to taste. Chill soup, covered, until very cold, at least 4 hours and up to 24.

　　

Step 3

Just before serving, stir in lemon juice and season with salt and pepper. Garnish soup with sour cream and herbs.