Comes from Cooking Light -- July 2007 issue, however I've modified it slightly. Swiss can be substituted however it will not be quite as rich. To obtain uniformly thin potato slices use a food processor or a mandoline.
Combine first 4 ingredients, stirring with a whisk, set aside.
Heat butter in a medium saucepan over medium heat. Add onion; cook 3 minutes or until tender, stirring frequently. Stir in milk mixture; bring to a boil. Cook for 2 minutes, stirring constantly. Remove from heat; add 3/4 cup cheese, stirring until smooth.
Arrange half of potatoes in a 11 x 7 inch baking dish coasted with cooking spray.Pour 1/2 of sauce over potaotes; top evenly with ham. Top with remaining potatoes and remaining sauce; sprinkle evenly with remaining 1/2 cup cheese.
Cover with foil coated with cooking spray.
Bake a 350 degrees for 40 minutes.
Uncover and bake an additional 45 minutes until potatoes are tender and the cheese is golden group.
Remove from oven; let stand 5 minutes before serving.