In 4 to 6-quart slow cooker, layer onion, beef and mushrooms. In small bowl, combine beef base, pepper, nutmeg and sherry; mix well. Pour over beef mixture.
Cover; cook on low setting for 8 to 10 hours.
About 20 minutes before serving, pierce potatoes several times with fork. Place on microwave-safe paper towel in microwave. Microwave on HIGH for 10 to 13 minutes or until tender, turning potatoes over and rearranging halfway through cooking. Let stand 5 minutes.
Meanwhile, combine sour cream, flour and ketchup; blend well. Stir into beef mixture. Increase heat setting to high; cover and cook an additional 5 to 10 minutes or until slightly thickened.
To serve, lightly roll potatoes on counter to soften center. Cut potatoes in half lengthwise; place on individual serving plates. If desired, sprinkle with salt. Spoon beef mixture over potatoes. Sprinkle with parsley.