Parsnip Yam and Watercress Chowder

Name: Parsnip, Yam, and Watercress Chowder
Ingredients: 1/4 cup (1/2 stick) butter; 4 cups 1/2-inch cubes peeled parsnips (about 4 large); 1 1/2 cups chopped onion (about 1 large); 3 cups (or more) low-salt chicken broth; 1 cup chopped peeled Granny Smith apple (about 1 medium); 1 1/2 cups 1/2-inch cubes peeled yam (red-skinned sweet potato; about 1 large); 1/8 teaspoon freshly ground nutmeg; 1/4 cup whipping cream; 4 cups watercress sprigs (tops of 2 bunches)