1 tablespoon butter
3 leeks (white part only), chopped
1 1/2 pound zucchini, peeled and diced
4 cups chicken stock
1 bunch of watercress, tough stems removed
1/3 cup heavy cream
salt and freshly ground pepper
In a large heavy saucepan, melt the butter over moderately low heat. Add the leeks and cook until softened but not browned, 5 to 7 minutes. Add the zucchini, increase the heat to moderately hight and saute for 2 minutes without browning.
Add the stock and bring to a boil. Reduce the heat to moderate and simmer until the zucchini is just tender, about 5 minutes. Add the watercress and simmer for 1 minutes longer. Using a food processor or blender, puree the soup, in batches, until smooth. Add the cream, if desired and season with salt and pepper to taste.