Combine all the ingredients, except parsley, in a slow cooker. Cook on low for a full 24 hours. 10 minutes before finishing the stock, add parsley (if using). Turn off slow cooker and skim any particles that have floated to the top. Allow to cool. Strain chicken parts and store in a freezer-safe bag for future stock. Discard vegetables. Chicken stock can be stored in the fridge for up to 5 days, or in the freezer for several months.
* Recipe can be made with only one chicken; simply halve the rest of the ingredients.