Snap off woody ends of asparagus; cut stalks into small pieces.
In large heavy saucepan, melt butter over medium heat.

Add leek and onion and saute, stirring occasionally for 10 minutes.

Add asparagus, lemon juice and broth and bring to boil; reduce heat and simmer until asparagus is tender, about 5 minutes.

In batches in blender or food processor, puree soup.

For an even smoother texture, press through sieve over saucepan to remove any fiber.

Stir in the cream and heat for two minutes.

Serve.