Gather the ingredients.

Mix together salt, paprika, onion powder, thyme, white pepper, black pepper, cayenne pepper, and garlic powder in a small bowl; set aside.

Remove and discard giblets from chicken. Rinse chicken cavity; pat dry with paper towels. Rub each chicken inside and out with spice mixture. Place 1 onion into the cavity of each chicken. Place chickens in resealable plastic bags or double wrap with plastic wrap. Refrigerate overnight, or at least 4 to 6 hours.

Preheat the oven to 250 degrees F (120 degrees C). Place chickens in a roasting pan.

Bake uncovered until an instant-read thermometer inserted into the meat near the bone reads at least 165 degrees F (74 degrees C), about 5 hours. Let chickens rest for 10 minutes before carving.