1. Combine the milk, chopped tarragon, and salt in a large saucepan and bring to a simmer (do not boil). Remove from the heat and let stand for one hour.
2. When the milk is ready, puree the avocado and lemon juice until smooth and whisk into the milk. Stir in the cayenne.
3. Chill at least 2 hours to marry the flavors. Then serve, or slowly reheat to enjoy hot. Garnish the soup with a swirl of Greek yogurt and fresh tarragon leaves.