Preheat your oven to 350°F (175°C) and grease a 2-quart casserole dish.

Layer half of the sliced potatoes at the bottom. Then add half of the cooked ham and chopped onions.

Repeat the layering with the remaining potato slices, ham, and onions.

In a medium saucepan, melt 3 tablespoons of butter over medium heat. Stir in 3 tablespoons of flour and cook for 1 minute.

Gradually whisk in 2 cups of milk and simmer until it thickens, about 5–7 minutes. Season with salt, pepper, and paprika.

Remove from heat and stir in 2 cups of shredded cheddar cheese until melted.

Pour the cheese sauce evenly over the layered ingredients in the casserole dish.

Cover the dish with aluminum foil and bake for 60 minutes.

Uncover, sprinkle with ½ cup of Parmesan cheese, and bake for an additional 20–30 minutes until golden-brown.

Let the dish rest for about 5–10 minutes before serving.