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 to 5 hours, or on HIGH for 2 to 3 hours, until the pork tenderloin is very tender and reaches an internal temperature of at least 145°F (63°C) when checked with an instant-read thermometer at the thickest part.
Once done, transfer the pork tenderloin to a cutting board and let it rest for about 5 minutes. While it rests, give the sauce in the slow cooker a quick stir to combine any separated juices into a smooth, savory gravy.

Slice the pork tenderloin into 1/2-inch thick medallions. Arrange the slices on a serving platter or individual plates and spoon the warm onion-mushroom gravy over the top. Serve immediately with your favorite sides.
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