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 5 cups broth, thyme, 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. 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 cream. Stir in vermouth and a squeeze of lemon juice. Season with salt and pepper to taste.
Garnish with asparagus tips. Serves 4-6. |