In a medium bowl, whisk together the dry onion soup mix, condensed cream of mushroom soup, and water until mostly smooth. A few small lumps are fine; they will dissolve as the mixture heats in the slow cooker.
Pour the soup and onion mixture evenly over the pork tenderloin, making sure the top is well coated. Some of the mixture will run down and pool around the meat, which will become the base of your savory gravy.
Cover the slow cooker with the lid and cook on LOW for 4
