Arrange potatoes and onion in bottom of slow cooker. Place pork on top of potatoes and onion. Spread garlic all over pork.
In a medium bowl, whisk together water, ketchup, vinegar, sugar, soy sauce, mustard, black pepper and salt. Pour mixture over pork.
Cover and cook on LOW for 6 to 8 hours or on HIGH for 3 to 4 hours.
Slice pork crosswise into thin slices and serve 12 ounces for this meal. Serve with all of potatoes, onions, and sauce.