Cream of Asparagus Soup

2 pounds asparagus, I prefer green and thin
1 large onion, chopped
3 T butter
5-6 cups chicken broth
1/2 cup heavy cream
1/4-1/2 t. fresh lemon juice
Cut tips from 12 asparagus 1 1/2 inches from top (if they are thick asparagus cut them in half lengthwise)These will be reserved for garnish.
Cut stalks and remaining asgaragus into 1/2 inche pieces.
Cook onion in 2 T butter in a 4 qt. heavy pot over moderately low hear, stirring until softened.  Add asparagus pieces and salt and pepper to taste, then cook, stirring, 5 minutes.  Add 5 cups broth and simmer, covered until asparagus are very tender, 15-20 minutes.
While soup simmers, cook reserved asparagus tips in boiling salted water about 3 minutes.  Drain
Puree soup with immersion blender or pour small batches in food processor.  Stir in heavy cream, then add more broth to thin soup to desired consistency.  Adjust seasonings and whisk in remaining 1T butter.
Add lemon juice and garnish with asparagus tips.
Makes 4-6 servings