Spray the inside of a 6 quart oval slow cooker, or similar with cooking spray.
Vegetables: Arrange the sliced vegetables and garlic cloves on the bottom of the slow cooker. Season lightly with salt and black pepper to your taste. Reserve 1-2 onion wedges for inside the chicken.
Chicken: Pat dry the chicken with paper towels. Rub inside and out with 3 tablespoons of softened butter. (You could also drizzle with olive oil, if preferred.)
Season the chicken inside and out with rotisserie chicken seasoning, garlic salt and black pepper, running your fingers under the skin to loosen seasoning under the skin of the breast and legs.
Place the reserved onion wedges and 2 sprigs of fresh thyme inside of the chicken. Center the chicken over the vegetables.
Dot the chicken with the remaining 1 tablespoon of butter. Secure the lid tightly on the slow cooker.
Cook on high for 4 hours or until the juices run clear. Using a meat thermometer 165-170°F inserted into the thigh.
Remove chicken to a platter or pan and allow to rest for 15 minutes before carving and serving.
Optional Broil: To crisp the skin, carefully the chicken remove to an oven safe dish. Place under the oven broiler for 3-5 minutes or until skin is golden. Rest for 15 minutes, then carve and serve drizzled with the jus from the slow cooker.