1 pound green asparagus
1 medium yellow onion, chopped
3 tablespoons unsalted butter
3 cups chicken broth
1/2 cup crème fraiche
1/3 cup heavy cream
A squeeze of fresh lemon juice
Salt and pepper, to taste
Cut tips from asparagus 1 1/2 inches from top and halve tips lengthwise if thick. Reserve for garnish. Cut stalks and all remaining asparagus into 1/2-inch pieces.
Cook onion in butter in a 4 or 6-quart heavy pot over moderately low heat, stirring, until softened. Add asparagus pieces and salt and pepper to taste, then cook, stirring, 5 minutes. Add chicken broth and simmer, covered, until asparagus is very tender, 15 to 20 minutes.
While soup simmers, saute asparagus tips in butter over medium heat.
Purée soup in batches in a blender until smooth. If you want a very creamy texture, you can put the purée through a food mill or press it through a sieve. Transfer to a bowl (use caution when blending hot liquids), and return to pan. Stir in crème fraiche and cream. Finish with lemon juice to brighten the soup. Season with salt and pepper to taste.
Garnish with asparagus tips and serve immediately.