Cut tips from 12 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 2 tablespoons butter in a 4-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 5 cups broth and simmer, covered, until asparagus is very tender, 15 to 20 minutes.
While soup simmers, cook reserved asparagus tips in boiling salted water until just tender, 3 to 4 minutes, then drain.
Purée soup in batches in a blender until smooth, transferring to a bowl (use caution when blending hot liquids), and return to pan. Stir in crème fraîche, then add more broth to thin soup to desired consistency. Season with salt and pepper. Bring soup to a boil and whisk in remaining tablespoon butter.
Add lemon juice and garnish with asparagus tips.
• Soup keeps, covered and chilled, 2 days. If making ahead, add last tablespoon butter and lemon juice after reheating.