Preheat oven to 350 degrees F (175 degrees C). Spray a 9×13-inch baking dish with cooking spray.
Bring a large pot of water to a boil. Add sliced potatoes and cook until almost tender, 5 to 10 minutes. Transfer to a colander and rinse with cold water until cool enough to handle. Drain well.
Melt butter in a large skillet over medium heat. Add onion; cook and stir until tender, about 5 minutes. Stir in flour, Dijon mustard, salt, and pepper; cook for 1 minute. Gradually pour in milk, stirring constantly until sauce is thickened, about 5 minutes. Stir in Cheddar cheese until melted.
Layer half of the potatoes in the bottom of the prepared baking dish. Top with 1 1/2 cup diced ham and half of the Cheddar cheese sauce. Repeat layering potatoes, ham, and sauce.
Bake in the preheated oven until the top is lightly browned, about 1 hour.
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