Follow these step-by-step instructions to create this delicious dish:

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Prepare the Potatoes: Wash and peel the potatoes. Use a mandoline slicer or a sharp knife to slice them into 1/8-inch thick rounds. Set aside.
3. Cook the Onion: In a large skillet, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
4. Make the Sauce:
   • Sprinkle the flour over the onions, stirring constantly for about 1 minute.
   • Gradually add the milk, whisking continuously until the mixture thickens and becomes smooth, about 3-4 minutes.
   • Stir in the shredded cheddar cheese, salt, black pepper, garlic powder, and paprika. Continue to stir until the cheese is completely melted and the sauce is creamy.
5. Layer the Dish:
   • Grease a 9×13-inch baking dish with butter or non-stick spray.
   • Arrange half of the sliced potatoes evenly in the bottom of the dish.
   • Sprinkle half of the diced ham over the potatoes.
   • Pour half of the cheese sauce over the layer of potatoes and ham.
   • Repeat the layers with the remaining potatoes, ham, and cheese sauce.
6. Bake:
   • Cover the baking dish with aluminum foil.
   • Bake in the preheated oven for 45 minutes.
   • Remove the foil and bake for an additional 20-25 minutes, or until the potatoes are tender and the top is golden brown.
7. Garnish and Serve: Remove the dish from the oven and let it sit for about 10 minutes before serving. Garnish with freshly chopped parsley for a burst of color and flavor.