Snap off tough lower parts of asparagus stems. Cut remaining asparagus into 1/2" pieces.

Melt butter in a 4-quart heavy pot over moderately low heat. Add chopped onion and cook, stirring, until softened.

Add asparagus pieces, salt and pepper to taste. Cook, stirring, 5 minutes.

Add broth and simmer, covered, until asparagus is very tender, 15 to 20 minutes.

Add soup in batches to a blender, along with chives, reserving a few stems for garnish. Purée soup until desired consistency.

Return to pan. Mix a small amount of soup into the crème fraîche to eliminate lumps. Add the crème fraîche mixture to the sop. If the soup is thicker than desired, a little more broth may be added. Season with salt and pepper.

Heat soup without boiling.

Garnish with a swirl of crème fraîche and a sprig of chive.

Serves 6.