Prepare the Casserole Dish:

1. Preheat the oven to 350°F (175°C).
2. Grease a 2-quart casserole dish with butter or cooking spray.

2. Layer the Ingredients:

1. Arrange half of the sliced potatoes in the bottom of the prepared dish.
2. Layer half of the diced ham and half of the chopped onion over the potatoes.
3. Repeat with the remaining potatoes, ham, and onion.

3. Make the Cheese Sauce:

1. In a medium saucepan, melt the butter over medium heat.
2. Stir in the flour and cook for 1 minute to eliminate the raw flour taste.
3. Gradually whisk in the milk, stirring constantly to create a smooth sauce.
4. Simmer until the sauce thickens, then season with salt, pepper, and paprika.
5. Remove from heat and stir in the cheddar cheese until melted and smooth.

4. Assemble the Dish:

1. Pour the cheese sauce evenly over the layered potatoes, ham, and onion.
2. Cover the dish with aluminum foil.

5. Bake the Casserole:

1. Bake in the preheated oven for 60 minutes.
2. Remove the foil, sprinkle Parmesan cheese on top, and bake uncovered for an additional 20-30 minutes, or until the top is golden and bubbly.

6. Cool and Serve:

1. Let the dish cool for 5-10 minutes to allow the sauce to set.
2. Serve warm and enjoy the comforting flavors.