Place the 4 tablespoons of butter, garlic powder, onion powder, paprika, thyme, salt and pepper in a bowl. Stir until combined.
Loosen the skin of the turkey breast. Spread part of the butter mixture under the skin of the turkey and spread some all over the top of the turkey, and spread some over the turkey legs.
Coat a large oval slow cooker with cooking spray and line the bottom of the slow cooker with the onion slices.
Place the turkey breast and legs on top of the onion slices. Cover and cook on low for 6-7 hours or until a thermometer inserted into the thickest part of the breast registers 165° F.
Transfer the turkey to a foil covered baking sheet. Preheat the broiler.
Broil the turkey for 4-5 minutes or until skin is browned and crispy.