Mix Water And Cream Of Tartar In A Large Bowl. Stir Potatoes Into Water Mixture Until Potatoes Are Well Coated; Drain. Layer Potatoes And Onion In A Slow Cooker; Season With Salt And Black Pepper. Heat Milk And Flour In A Saucepan Over Medium Heat, Stirring Often, Until Boiling, About 5 Minutes; Season With Salt And Black Pepper. Pour About Half The Milk Sauce Over The Potato Mixture. Scatter Cheddar Cheese And Ham Into The Slow Cooker; Cover With Remaining Milk Sauce. Cook On Low Setting For 9 To 10 Hours Or On High For 4 1/2 To 5 Hours.