Cook onion in olive oil in a 4-quart heavy bottom pot over medium-low heat, stirring, until softened. Add asparagus pieces and salt and pepper to taste, then cook, stirring, for 5 minutes. Add chicken broth and simmer, covered, until asparagus is very tender, about 15-20 minutes.
While soup simmers, cook reserved asparagus tips in boiling salted water until just tender, 3 to 4 minutes. Drain.
Puree soup in batches in a blender (or use an immersion blender, my choice) until smooth. Return soup to pan.
Stir in sour cream. Season with salt & pepper. Bring soup to a boil and whisk in butter.
Add lemon juice and garnish with asparagus tips.
Recipe by Faithfully Gluten Free at http://www.faithfullyglutenfree.com/2011/04/creamy-asapargus-soup.html