Preheat the oven to 350°F (175°C).

In a medium saucepan, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.

Cube the leftover ham and russet potatoes into 1-inch pieces.

Bring a pot of salted water to a boil and add cubed potatoes. Boil for about 5 minutes until starting to soften, then drain.

In the same saucepan, whisk in flour, cooking for 1 minute. Gradually add milk, whisking until thickened, around 3-4 minutes.

Season the sauce with salt, pepper, ground mustard, and garlic powder.

In a large mixing bowl, combine ham, boiled potatoes, and cheddar cheese. Pour the sauce over and toss to coat.

Transfer to a greased baking dish and sprinkle reserved cheddar cheese on top.

Bake for about 45 minutes until bubbly and golden brown.

Let the casserole rest for 10-15 minutes before serving.