If you have a regular crock pot, cook on low heat 2 hours. Whisk together water with corn starch (or arrow root powder) and add to crock pot. Stir. Turn crock pot to 'high' and cook for one more hour.
If you have a crock pot like mine that has a sauté/browning feature you can cook for 3 1/2 hours on low, add corn starch mixture and press brown/sauté, adjust the temperature 425. Cook for 15 more minutes till sauce is thick.