In a Dutch oven, heat the olive oil over medium heat. Add the onions and garlic and cook, stirring, until the onions are translucent. Add the broth and potatoes and simmer until potatoes are soft, about 25 minutes. Add the watercress and simmer 5 more minutes. Let cool, then puree in batches in a blender or food processor. Add the cream, and season with salt and pepper to taste. Cover and chill overnight. Stir in the lemon juice. Serve sprinkled very lightly with nutmeg.
Makes 6 servings