1. Snap off and discard tough ends of asparagus. Cut into 2-inch pieces.
2. Combine asparagus, broth, onion, garlic, and 1/2 tsp. thyme in a large saucepan over medium-high heat; bring to a boil. Reduce heat to medium-low; cover and simmer 10 minutes. Remove from heat; let stand 10 minutes. Process asparagus mixture, in batches, in a blender or food processor until smooth. Return to pan.
3. Whisk together flour and milk until smooth. Gradually add flour mixture to asparagus mixture, whisking until blended. Bring to a boil, stirring constantly. Reduce heat; simmer, stirring constantly, 5 minutes. Remove from heat; stir in butter, salt, 1/4 tsp. lemon rind, and remaining 1/4 tsp. thyme.
4. Combine sour cream, lemon juice, and remaining 1/4 tsp. lemon rind. Top each serving with about 5 tsp. sour cream mixture. Garnish, if desired.
Note: Freeze soup after Step 3 for up to 1 month. When ready to serve, thaw, reheat, and continue with Step 4.