In a crock pot, layer potatoes, onions and ham. In a saucepan, melt butter. Stir in flour, mustard, salt and pepper until smooth. Combine soup and water; gradually stir into flour mixture. Bring to a boil; cook and stir for 2 minutes or until thickened and bubbly. Pour over ham. Cover and cook on low for 8-9 hours or until potatoes are tender. If desired, sprinkle with cheese before serving.
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