Preheat your oven to 350°F (175°C) and grease a 9×13-inch casserole dish.

In a large pot, place the cubed potatoes and cover with salted water. Bring to a boil, then reduce to medium-low and simmer until just tender, about 10–15 minutes. Drain and set aside.

Meanwhile, melt the butter in a medium saucepan over medium heat. Whisk in the flour to form a paste (roux), cooking 1–2 minutes until bubbly.

Gradually whisk in the milk, stirring constantly until the sauce thickens, about 3–5 minutes. Season with salt and pepper, then remove from heat.

Stir 1½ cups of the shredded cheddar and the sour cream (if using) into the sauce until smooth.

In a large bowl, gently combine the cooked potatoes, diced ham, chopped onion, and creamy sauce.

Transfer the mixture to your prepared casserole dish. Sprinkle the remaining ½ cup cheddar and the crushed crackers or breadcrumbs on top.

Bake for 25–30 minutes, until bubbly and golden brown on top. Let rest 5 minutes, garnish with parsley, then serve warm.