Step 1

In a large (4-quart) saucepan, melt butter over medium heat. Add shallots and thyme; cook until shallots are soft, about 5 minutes. Add flour; cook until incorporated, stirring constantly, about 1 minute.

Step 2

Add asparagus and 6 cups water; season generously with salt. Cover, and bring to a boil.

Step 3

Reduce heat; cover, and simmer, until asparagus is bright green and just tender, 2 to 4 minutes. Working in small batches, purée soup in a blender until very smooth. To prevent splattering, fill blender only halfway, and allow heat to escape: Remove cap from hole in lid, and cover lid with a dish towel. (To freeze, see note.) If serving right away, stir in cream and lemon juice.