In a saucepan, melt the butter over medium-high heat. Add the onions and cook, stirring, until softened, about 4 minutes. Add the garlic and cook until fragrant, about 1 minute. Add the zucchini, salt, and pepper, and cook until the zucchini is tender, stirring occsionally. Add the stock and thyme and bring to a boil. Reduce the heat and simmer, stirring occasionally, for 20 minutes.

Add the watercress, remove from the heat, and let the watercress wilt, about 1 minute. Discard the thyme sprig.

With a hand-held immersion blender or in batches in a food processor, puree the soup until very smooth. Return to the heat and stir in the cream. Taste and adjust the seasoning, keeping in mind, that salt is less evident in cold foods.

Let the soup cool and then refrigerate until well chilled, at least 4 to 6 hours.

Serve the soup in small bowls, with a dollop of crême fraiche and watercress sprig to garnish each bowl. Serve with the cheese crackers.