Layer potatoes, onion, 1 teaspoon salt and ham into slow cooker. Melt butter in saucepan. Stir in flour and 1 teaspoon salt. Cook until bubbly. Gradually add milk. Cook until smooth and thickened. Add cheese and stir until melted. Pour over potato-ham mixture, stirring lightly. Cover. Cook on low 6 to 7 hours, or high 3 to 4 hours.