1. Wash watercress and dry on paper toweling. Set aside 6 sprigs for garnish; chop remaining stems and leaves fine. (You should have about 3 cups.)
2. Smooth flour and about 1/4 cup of the milk to a paste in a large saucepan; slowly stir in remaining milk, onion and salt. Cook, stirring constantly, until mixture thickens slightly and bubbles 3 minutes. Remove from heat. Stir in chopped watercress.
3. Ladle soup into heated soup plates; float a watercress sprig on each serving.