In a 4- to 5-quart slow cooker combine onion, celery and carrots. Place turkey on vegetables. Add broth, water and pepper to cooker.
Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours.
If using low-heat setting, turn to high-heat setting while preparing the dumplings. For dumplings, in a large bowl combine flour, parsley, sage, thyme, margarine, milk and egg. Stir just until moistened. Turn dough out onto a lightly floured surface and knead just until dough is smooth.
On a lightly floured surface, roll dough into a 10x8-inch rectangle, about ¼ inch thick. Using a pizza cutter, cut the dough into 1-inch-wide strips; cut the strips into 1-inch squares. Set aside.
Remove turkey from slow cooker. Using two forks, shred the turkey into large pieces. Return turkey to cooker. Slowly add the dumplings, a few at a time, stirring so they won't stick together. Cover and cook on high-heat setting for 30 to 40 minutes more or until dumplings are cooked but tender. The liquid in the slow cooker will thicken as the dumplings cook. Carefully spoon soup into shallow dishes.
Tips: If you don't have leftover roasted turkey breast, substitute 1 pound skinless, boneless chicken thighs that have been browned in a nonstick skillet coated with nonstick cooking spray.
For easy cleanup, line your slow cooker with a disposable slow cooker liner. Add ingredients as directed in recipe. Once your dish is finished cooking, spoon the food out of your slow cooker and simply dispose of the liner. Do not lift or transport the disposable liner with food inside.