Wash and trim the watercress. Pat it dry and chop it (you should have about 1.5 cups). Put the watercress in the bowl of an electric blender with the sour cream, lemon juice, Tabasco sauce, and salt and pepper. Turn the blender on high for several seconds, or until the mixture is very smooth. Serve the sauce over the trout. Garnish each serving with a lemon wedge and sprig of watercress. YIELD: 6 servings. (NASON'S NOTE: Try this sauce with leftover cooked and chilled East Lake "pink-meat" trout if smoked trout is not available.)