Preheat the oven to 400. I used convection. Cut the onion in half. Lightly spray the vegetables with oil, arrange on a baking sheet with the bones and roast for about 30 minutes or until the veggies start to brown.
Take the veggies out in this order while continuing to brown the bones and add them to the pot of water with the dried herbs: celery, carrots, onions. The celery will be the the first to finish. Continue to roast the carrot, adding it to the pot when it starts to carmelize.
Flip the onions and the bones at this time and continue to roast for another 15 to 20 minutes depending on how well the bones and onion have browned. Add all this, including the juices to the pot of water and herbs.
Boil everything together for about 4 hours or until the water has reduced by half on medium high heat. Cooking it longer won't hurt. Let it cool, strain, and store.