Line a roasting dish with the onion and then the chicken backs. Put into a 350 degree oven for at least an hour and as much as two hours. The backs want to be well-browned and the dripping not boiling with water in the bottom. Drain the fat into a heatproof measure and boil the browned backs for stock. The fat that you get will stay liquid above 70 degrees and is pleasant for all uses. The stock you get will be rather dark and full bodied.