Begin by assembling all necessary ingredients.
In a small bowl, combine the salt, paprika, garlic powder, onion powder, dried thyme, white pepper, black pepper, and cayenne pepper. Stir thoroughly to create an even seasoning blend.
Remove and discard the giblets from the chickens. Rinse the cavities under cold water and pat them dry with paper towels. Generously rub the seasoning mix onto the exterior and interior of each chicken. Place one quartered onion inside the cavity of each bird.
Seal the seasoned chickens in resealable plastic bags or tightly wrap them in plastic wrap. Allow them to marinate in the refrigerator for at least 4 to 6 hours, or preferably overnight for deeper flavor penetration.
Preheat your oven to 250°F (120°C). Arrange the chickens in a roasting pan, ensuring they are uncovered.
Roast the chickens until an instant-read thermometer, inserted into the thickest part of the meat near the bone, registers a minimum internal temperature of 165°F (74°C). This process should take approximately 5 hours.
Once cooked, remove the chickens from the oven and let them rest for 10 minutes before carving. Serve and enjoy the perfectly roasted, tender meat.