Pat the chicken dry and season it inside and out with the salt, black pepper, garlic powder and thyme.
Put the onion halves cut-side down in a 6-quart slow cooker and arrange the carrots, parsnips and potatoes around them.
Pour the broth over the vegetables and set the chicken breast-side up on the onion.
Cover and cook on low until the breast reaches 165°F and the thigh reaches at least 175°F, 5 to 5 1/2 hours.
Transfer the chicken carefully to a foil-lined pan and move the vegetables to a serving dish.
Broil the chicken 6 inches from the heat for 3 to 5 minutes until the skin browns, watching continuously.
Rest the bird for 15 minutes before carving and serve it with the vegetables and skimmed juices.