Stir the sweet potatoes, brown sugar, cinnamon, nutmeg and milk in a medium bowl. Spoon the sweet potato mixture around the inside edge of a 10-inch round casserole, forming a ring.
Heat the stock and onion in a 10-inch skillet over medium-high heat to a boil. Reduce the heat to low. Cook until the onion is tender, stirring occasionally. Stir in the soup, turkey and water. Cook until the mixture is hot and bubbling. Spoon the turkey mixture into the center of the sweet potato ring.
Bake at 350 degrees F for 30 minutes or until the turkey mixture is hot and bubbling. Sprinkle with the walnuts and parsley, if desired.