Preheat the oven to 350°F (175°C). Grease the baking dish with butter or cooking spray.

Melt the butter in a medium saucepan over medium heat, then sauté the chopped onion until translucent (4-5 minutes).

Cut the leftover ham into 1-inch cubes. Peel and cube the potatoes, then parboil them in boiling water for 5 minutes before draining.

Sprinkle the flour over the sautéed onions and stir for 1 minute. Gradually whisk in the milk, cooking until thickened (5-7 minutes). Season with salt, pepper, mustard, and garlic powder.

In a bowl, combine the ham, parboiled potatoes, and cheddar cheese. Pour the sauce over and stir to coat.

Transfer the mixture to the baking dish, top with reserved cheese, and bake for 45 minutes until golden brown and bubbly.

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