For the Crème Fraiche:
1 1/2 cups heavy or whipping cream
½ c Sour Cream
For the Soup:
1/4 cup (1/2 stick) unsalted butter or 2T oil and 2T butter
4 cups chopped leeks (white and pale green parts only; from about 2 large leeks)
3 pounds asparagus, tough ends broken off and discarded, spears cut into 2 inch pieces
6 1/2 cups reduced-sodium chicken broth
3 tablespoons chopped chives (sprinkle over soup as a garnish)
FOR THE CREME FRAICHE:
Whisk cream and sour cream in small bowl to blend. Cover and let stand in warm part of kitchen until thickened, at least 5 hours or overnight. Refrigerate.
FOR THE SOUP:
Melt butter in heavy large deep-sided nonreactive pot over medium heat. Add leeks and sauté until softened, about 5 minutes. Add asparagus and sauté 2 minutes. Add broth and 1 teaspoon salt. Bring to simmer, reduce heat and simmer gently, uncovered, until vegetables are very tender, about 15 minutes.
Using hand held blender, puree soup in pot. Alternately, working in batches, puree soup in food processor, blender, or food mill. Return soup to same pot. Stir in 1 cup crème fraiche. Season to taste with salt. If serving immediately, reheat soup a few minutes if necessary.
Crème fraiche can be made 1 week ahead. Keep refrigerated. Soup can be prepared 2 days ahead. Cool. Cover and refrigerate. Stir over medium heat until heated through before serving.
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