In a saucepan, melt the butter over medium-high heat. Add the onions and cook, stirring, for 3 minutes. Add the garlic and cook for 15 seconds. Add the zucchini, salt, and pepper, and cook until tender. Add the stock and thyme and bring to a boil. Reduce the heat and simmer, stirring occasionally, for 20 minutes.
Add the watercress and let wilt, about 5 minutes. Remove from the heat and discard the thyme sprig.
With a hand-held immersion blender or in batches in a food processor, puree the soup. Return to the heat and stir in the cream. Heat gently and cook for 5 minutes. Adjust the seasoning, to taste, keeping in mind, that salt is less evident in cold foods.
Let cool and refrigerate until well chilled, at least 4 to 6 hours.
Transfer to a large thermos. To serve, pour into double-handled cream soup bowls or decorative cups. Swirl creme fraiche into each serving and garnish with watercress sprigs and chives. Serve with cheese crackers.
Yield: 4 to 6 servings