- Combine milk, tarragon, and salt; heat until liquid just begins to simmer (do not boil).
- Remove from heat; let stand 1 hour.
- Meanwhile, puree avocado and lemon juice until smooth.
- When milk is ready, whisk avocado mixture into milk mixture.
- Stir in cayenne.
- To serve hot, heat until liquid just begins to simmer.
- To serve cold, chill at least 2 hours to marry flavors.
- Serve the soup the day it's made.
- Garnish each serving with a swirl of sour cream.