Place pork chops in 2 to 3 1/2-quart slow cooker. Top each with onion slice. Sprinkle with garlic. Pour barbecue sauce over pork.
Cover; cook on low setting for 5 to 6 hours.
About 5 minutes before serving, remove pork from slow cooker; place on serving platter. Top each chop with onion slice; cover to keep warm.
In 2-cup glass measuring cup or small microwave-safe bowl, blend water and cornstarch until smooth. Pour juices from slow cooker into cornstarch mixture; mix well. Microwave on HIGH for 1 to 2 minutes or until mixture boils and thickens slightly, stirring once halfway through cooking. Serve sauce with pork.