1. In a small bowl, mix together salt, paprika, onion powder, thyme, white pepper, black pepper, cayenne pepper, and garlic powder. Remove and discard giblets from chicken. Rinse chicken cavity, and pat dry with paper towel. Rub each chicken inside and out with spice mixture. Place 1 onion into the cavity of each chicken. Place chickens in a resealable bag or double wrap with plastic wrap. Refrigerate overnight, or at least 4 to 6 hours.
2. Preheat oven to 250*F.
3. Place chickens in a roasting pan. Bake uncovered for 5 hours, to a minimum internal temperature of 180*F. Let the chickens stand for 10 minutes before carving.
MY NOTE: Made this last night for 2nd time with some minor changes, and it was SO good - We said it was possibly the best chicken I ever made and as good, if not even better than the store rotisserie chickens (which, in our opinion, are extremely yummy). The only things I did differently were I used a large 7+ lb. roaster instead of the 2 smaller ones. For the spices, I eliminated white pepper and cut the cayenne and paprika in half, and increased the garlic powder to a full teaspoon. Even doing that there is still plenty of kick in the spice mix. Like another reviewer I added a couple Tbsps. of minced garlic to the onions in the cavity. The biggest change? Last time I just seasoned and stuck in oven. This time, I wrapped in double plastic wrap (with the onions/garlic in place in the cavity) & ended up marinating for over 24 hours in the fridge - purely by accident, but it made all the difference! Slow-roasting at 250 (I did go up to 275 for about 45 minute in middle of cooking to ensure it would be done in time for dinner) cooked the bird perfectly in exactly 4 hours (Use a meat therm. - take out at 170, and let it rest before carving, it will be perfectly cooked but not dried out). Made gravy with the drippings, but the white meat was so moist it didn't even need it. I may never make a roast chicken any other way! P.S. when the chicken is done, drizzle some pan drippings over it, the chicken will suddenly glisten and look picture perfect for the table.
My only suggestion is to get a jar and mix up a lot of the seasonings at one time to keep in your cabinet. I changed the servings to 64 and the ingredents came up in cup and tablespoon amounts for easy mixing. Then when I make a chicken I have the "mix" at my fingertips. It takes 3 to 4 tablespoons of mix per chicken. I normally serve this "rubber chicken" style with lots of veggies so I have leftover chicken. The next day I make enchiladas with the leftovers. Then I boil the bones for tortilla soup. This gives 3 meals my family of 5 loves. Even my picky one eats these 3 meals. I normally make this in the crockpot for easy cooking, but the roasting it in the oven yeilds better results on wet days.