We prefer the richness of Gruyère for this gratin, but you can also substitute any type of Swiss cheese. Try a mandoline or the slicing blade of a food processor to create uniformly thin potato slices. You can use a skewer to check the tenderness of the potatoes as the gratin bakes.
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 1/2 cup cheese, stirring until smooth.
Arrange half of potatoes in an 11 x 7-inch baking dish coated with cooking spray. Pour half of sauce over potatoes; 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 at 350° for 40 minutes. Uncover and bake an additional 45 minutes or until potatoes are tender and the cheese is golden brown. Remove from oven; let stand 5 minutes before serving.
I took this to a small dinner party and it seemed to be a hit. I used 1/4 cup of flour to make the sauce thicker as other had noted it seemed watery. I think that was about right. I did add about 1/2 cup of shredded cheddar to the top for the last 45 minutes of baking time and it really seemed to add a nice touch of flavor. I would make this again and add a clove of minced garlic to the onion for a little more flavorful sauce.
Very good, but then I used half and half instead of 1% milk in mine. Also, I used Cheddar, hubby doesn't care for Gruyere. I make scalloped potatoes with ham frequently and this was the first time I made the white sauce. Usually I just put all the ingredients in layering the potatoes, ham, & onions, and sprinkling flour over each layer & add dabs of butter then add the half and half - I don't put cheese in mine. I know my method is not very low fat but it's deliciously creamy. This recipe tasted pretty close.
My four-year-old daughter insists she doesn't like potatoes, but asks for seconds and even thirds of this dish, discreetly referred to as simply "gratin" in our house. It's so satisfying, I usually serve it as the main course, rather than a side dish.