Step 1Heat butter over medium-low heat in medium saucepan. Add onion, cover and cook until onion is tender, about 5 minutes.
Step 2While onion is cooking, rinse watercress, and pull off and discard tough stems. Add watercress, broth and water to pot. Bring to boil, then reduce heat and simmer until watercress is very tender and flavors have melded, about 10 to 12 minutes.
Step 3Puree soup at high speed in blender until smooth, about 1 minute (puree in batches, if necessary). Return soup to pot and heat through. Add salt to taste, about 1/2 teaspoon, and lemon juice. Garnish soup with yogurt or sour cream.