Serve this inexpensive soup as a light lunch, or as an appetizer for a multi-course dinner.
Combine potatoes and water in soup kettle. Boil 20 minutes or until tender. Drain and save water. Press potatoes through food mill and return to kettle with water. Chop watercress (reserving a few whole leaves for garnish), add to kettle, and cook 5 minutes. Add butter and milk. Heat until warm, season with salt and pepper, and garnish and watercress leaves. (Also good cold.)