Directions

1. Preheat oven to 350F (175C).
2. Melt butter in saucepan over medium heat; add onion and cook until tender.
3. Stir in flour and cook 1-2 minutes.
4. Gradually whisk in milk to form a smooth sauce.
5. Add salt, pepper, and dry mustard; cook until thickened.
6. Layer half of potatoes in a greased casserole dish.
7. Add ham evenly over potatoes.
8. Pour half of sauce over potatoes and ham, then sprinkle half of the cheese.
9. Repeat layering with remaining potatoes, sauce, and cheese.
10. Cover with foil and bake for 50 minutes, then uncover and bake another 10 minutes until cheese bubbles.