Add all ingredients to a large resealable freezer bag.
Squish ingredients to combine, remove excess air, seal, and freeze.
Thaw (or can be cooked from frozen).
Cook in slow cooker on low for 5-6 hours.
Mix together the cornstarch and water in a small bowl and add the cornstarch slurry to the crockpot, allowing it to cook for an additional 10 minutes to thicken the gravy.