In a medium saucepan, melt the butter over medium heat.
Add the diced onions and sauté for 5 minutes until softened and translucent.
Add the sliced cucumber, vegetable stock, tarragon, vinegar, salt, and pepper to the pot.
Cover, bring to a boil, then reduce the heat and simmer for 20 minutes, or until the cucumbers are very tender.
Stir occasionally.
While the soup simmers, prepare the watercress.
Set aside a few perfect sprigs for garnish.
Remove the leaves from the remaining stems (discard stems) and finely mince the leaves.
Carefully transfer the hot soup to a blender or food processor in batches and purée until completely smooth.
Pour the puréed soup into a large bowl and stir in the minced watercress. Allow it to cool for at least 15 minutes.
Once cooled slightly, whisk in the sour cream until fully combined.
Cover and chill the soup in the refrigerator for at least 3 hours, or until ice cold.
Serve in chilled bowls, garnished with the reserved watercress sprigs.
