In a large baking dish, arrange the onion slices in an even layer. Place the chicken on top.
Combine the remaining ingredients and rub the seasonings over the chicken, then tent the chicken in foil and roast for 3 hours.
Remove the foil and baste the chicken with the juices at the bottom of the pan. Return to the oven for 1 hour.
Baste again, then return to the oven for 1 hour. Repeat until the chicken reaches 165 degrees F inside, if it hasn't already.
When the chicken is done, let rest 10 minutes. Meanwhile, pour the juices into a saucepan and bring to a simmer. Reduce by half, 8-12 minutes, and pour over chicken to serve.