In large bowl, combine 1 cup water and cream of tartar. Add sliced potatoes; toss to coat. Drain, discarding water. Place potatoes in 3 to 4-quart slow cooker.
In medium bowl, combine soup, 1/2 cup water, flour, butter, salt, pepper and garlic powder; mix well. Pour mixture over potatoes; stir gently to coat.
Cover; cook on Low setting for 5 to 6 hours.
Just before serving, sprinkle Parmesan cheese over top.