In a small bowl, mix salt, paprika, onion powder, thyme, white pepper, black pepper, cayenne pepper, and garlic powder.
Remove giblets from chickens. Rinse cavities and pat completely dry.
Rub seasoning mixture inside and outside of each chicken.
Place one quartered onion inside each chicken cavity.
Wrap chickens tightly and refrigerate for at least 4–6 hours or overnight.
Preheat oven to 250°F (120°C).
Place chickens in a roasting pan and roast uncovered for about 5 hours, or until internal temperature reaches 165°F (74°C).
Rest for 10 minutes before carving and serving.