Pat the chicken thoroughly dry inside and out.
Mix the salt, paprika, onion powder, garlic powder, thyme, black pepper, cayenne and vegetable oil into a paste.
Rub the paste all over the bird and set the onion quarters in a 6-quart slow cooker.
Place 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 1/4 hours.
Move the chicken to a foil-lined pan and broil 6 inches from the heat for 3 to 5 minutes until the skin browns.
Rest for 15 minutes before carving and save the skimmed cooking juices for leftovers.