Preheat your oven to 350°F (175°C) and prepare the baking dish by greasing it lightly.

Melt 3 tablespoons of unsalted butter in a medium saucepan over medium heat and sauté 1 chopped onion until translucent.

Peel and cube 2½ lbs of russet potatoes into 1-inch pieces, then parboil in a large pot of boiling water for about 5 minutes.

Sprinkle in 3 tablespoons of all-purpose flour, stirring for 1-2 minutes, then gradually whisk in 2 cups of whole milk until thickened.

In a large mixing bowl, combine the cubed ham and drained potatoes, add the shredded cheddar cheese and cheese sauce, and stir gently.

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

Bake for approximately 45 minutes until golden brown and bubbling at the edges.

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