In a large dutch oven or stock pot, heat the olive oil over medium high heat. Sear onion, cut side down until very dark.
Add chicken bones, juices from the cooked chicken, herbs, and fill to the top with water.
Simmer with lid on for 24 hours.
Once cool, remove bones, and drain stock over a fine mesh strainer. Pour into freezer containers or glass jars if using within a week. Keeps fresh in the fridge up to a week.