7. Pour the cheese sauce evenly over the layers of potatoes, ham, and onions in the casserole dish.
8. Cover the dish with foil and bake in the preheated oven for 60 minutes.
9. After 60 minutes, remove the foil, sprinkle the grated Parmesan cheese over the top of the casserole, and return it to the oven.
10. Bake uncovered for an additional 20-30 minutes, or until the top is golden brown and bubbly.
11. Let the ham and potato au gratin cool slightly before serving.
Enjoy your deliciously cheesy and comforting ham and potato au gratin!
