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.