Spray the inside of a 6 quart oval slow cooker, or similar with cooking spray.
Arrange the sliced vegetables and garlic cloves on the bottom of the slow cooker.
Season lightly with the same seasoning used for the chicken and black pepper to your taste. Reserve one or two wedges of onion to stuff into the bird.
Rinse and pat dry the chicken. Rub inside and out with 3 Tbsp softened butter or drizzle with olive oil, if preferred.
Season the chicken inside and out with the seasoning of choice, running your fingers under the skin to loosen and allow you to season under the skin of the breast and legs, too.
Place the reserved onion wedges and 2 sprigs of fresh thyme inside of the bird. Center the bird on top of the vegetables.
Dot with the remaining 1 Tbsp butter. Secure the lid.
Cook on high for 4 hours or until the juices run clear. Using a meat thermometer 165-170°F inserted into the thigh.
Rest for 15 minutes before serving.
To crisp the skin, carefully remove to an oven safe dish and place under the oven broiler for 3-5 minutes. Rest for 15 minutes, then carve and serve drizzled with the jus from the slow cooker