Season both sides of the cube steak and set aside.
In a slow cooker, combine the cream of mushroom soup, beef stock, and soup mix, stirring until well blended.
Add the cube steak to the slow cooker and coat it in the mixture.
Cover and cook on low for 5-6 hours or on high for 3-4 hours.
During the last hour of cooking, whisk together the water and cornstarch until fully dissolved to create a slurry.
Pour the slurry into the slow cooker and gently stir to combine.
Turn the heat to high and cook for an additional hour, or until the gravy has thickened.