Preheat your oven to 350°F (175°C).

In a medium saucepan over medium heat, melt the unsalted butter and sauté the chopped onion until soft and translucent, about 3-5 minutes.

Peel and cube the Russet potatoes into uniform 1-inch pieces; cube the leftover ham into bite-sized pieces.

In a large pot of boiling water, parboil the cubed potatoes for about 5 minutes. Drain and return to the pot.

Sprinkle in the flour over the sautéed onions and cook for 1-2 minutes. Gradually whisk in the milk and let thicken for about 3-5 minutes.

In a large bowl, combine the ham, parboiled potatoes, and shredded cheddar cheese. Pour over the creamy sauce and mix to coat.

Transfer the mixture to a greased baking dish and sprinkle the remaining cheddar cheese on top.

Bake in the preheated oven for approximately 45 minutes until golden brown and bubbling.

After baking, let the casserole rest for 10-15 minutes before serving.