Cover the slow cooker and cook on low heat for a minimum of 12 hours, but preferably 18-24 hours.
Remove the bones and herbs from the broth. Strain the broth with a fine mesh strainer to make sure everything has been removed.
Store the broth in air tight containers in the refrigerator for 3-4 days or frozen for up to 1 year. If you freeze your bone broth it is recommended to freeze the broth in measured proportions for easy use.
The majority of the carbs come from the onion. To lower the carb value, you may use onion flake or another flavouring.