Crème de Cresson Tourangelle

My eyes light up whenever I see watercress soup, with its brilliant green color and slightly acidic bite that is delicious hot or chilled. However, watercress must be blanched and then thoroughly simmered in broth before any milk is added, or the acid in the watercress will cause the milk to curdle. This piquant green can be difficult to cultivate, prospering in currents of clean river water. It flourishes around Tours, where this soup originated. For Cream of Spinach Soup, substitute