Preheat oven to 350 degrees F.
Combine cooked potatoes, cubed ham and minced onion in a 1 to 1½ quart casserole dish.
In a saucepan melt the butter on medium heat.
Add the arrowroot powder and mix until completely combined.
Add the milk, stirring until the mixture begins to thicken.
Add the cheese and pepper and mix until the cheese melts.
Pour cheese mixture over the potato mixture.
Bake for 40 minutes. Sauce should be bubbly.