Creamy Ham and Potatoes

In a slow cooker, 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.