Mix the seasoning: In a small bowl, combine salt, paprika, onion powder, thyme, pepper, garlic powder, and cayenne pepper.
Prepare the chicken: Remove giblets, rinse the chicken, and pat completely dry with paper towels. Rub the seasoning mixture all over, including under the skin if you can.
Stuff the cavity: Place the quartered onion and rosemary sprigs inside.
Marinate: Wrap tightly with plastic wrap or seal in a large resealable bag. Refrigerate overnight (or at least 6 hours) to let the flavors sink in.
Roast: Preheat oven to 250℉. Place the chicken in a roasting pan, uncovered, and bake for 5 hours, or until the internal temperature reached 180℉.
Rest: Remove from the oven, tent loosely with foil, and let rest for 10-15 minutes before carving.