Melt butter. Add flour and whisk in until lumps are gone. Continue stirring frequently while cooking over low heat until golden brown. Add milk and seasonings. Heat over low heat, stirring frequently until thickened.
Peel and slice potatoes into thin, even ovals. Prepare a 10x13 inch baking dish with baking spray. Layer potatoes, sauce, ham, and cheese. Repeat, and end the casserole with a layer of potatoes and cheese on top.
Bake at 325 for 1 hour, or until potaotes are tender and everything is melted and delicious. If the top starts to get too browned, cover loosely with tin foil.