refrigerated butter-flavored cooking spray

1	tablespoon (12.5 g) reduced-calorie margarine

1/2	cup (80 g) chopped onion

2	bunches watercress

8	cups (1.9 l) canned low-fat, low-sodium chicken broth

2	medium Russet potatoes, peeled and thinly sliced

salt (optional) and freshly ground pepper to taste

2	cups (480 ml) evaporated skim milk

skim milk (optional)

lemon slices for garnish (optional)

1. Lightly coat the bottom of a large saucepan with cooking spray. Add margarine and place over medium heat. Add onion and sauté until onion is limp, about 4 minutes.
2. Meanwhile, rinse watercress and drain on paper towels. Add the leaves and tender stems to the saucepan and sauté for 2 minutes.
3. Add chicken broth and potatoes. Cook, covered, until potatoes are tender, about 20 minutes.
4. Transfer mixture to a food processor or blender. Process until smooth. Return the mixture to the saucepan and season with salt (if using) and pepper. Stir in the evaporated milk and reheat, adding a bit of skim milk (if desired) to thin to desired consistency. Serve in wide, shallow soup bowls.
5. If desired, float a lemon slice on each serving.