1. Preheat oven to 350-375°F.
2. Grease a casserole dish with butter or cooking spray.
3. Thinly slice potatoes (about 1/8 inch) using a mandoline or sharp knife.
4. Dice ham into small bite-sized pieces.
5. In a saucepan, melt butter over medium heat.
6. Add diced onions and minced garlic, sautéing until soft and fragrant.
7. Stir in flour to create a roux, cooking for 1-2 minutes.
8. Gradually whisk in milk or cream, stirring continuously until thickened.
9. Add salt, pepper, and any additional seasonings or herbs as desired.
10. Remove sauce from heat and mix in shredded cheese until melted and smooth.
11. In the prepared casserole dish, layer sliced potatoes evenly.
12. Sprinkle diced ham over the potato layer.
13. Pour a portion of the cheese sauce over the ham and potatoes.
14. Repeat layering process 2-3 times until all ingredients are used.
15. Cover the dish with foil and bake for 30-50 minutes.
16. Remove foil and continue baking uncovered for another 25-35 minutes until potatoes are tender and golden.
17. Let rest for 5-10 minutes before serving.
18. Store leftovers in the refrigerator for up to 3-4 days.
19. Reheat in the oven or microwave before serving.
20. Optionally, freeze leftovers, noting that texture may change slightly after thawing.