Preheat the oven to 350°F. Place the ham in a roasting pan.
In a small saucepan, melt 1 tablespoon of the butter with the brown sugar over low heat until smooth. Brush the glaze evenly over the ham.
In a separate saucepan, melt the remaining 2 tablespoons of butter. Whisk in the flour, salt, and pepper, then slowly add the milk while stirring until the sauce thickens.
Layer the sliced potatoes and onion in a greased casserole dish. Pour the sauce over the top.
Place both the ham and the casserole dish in the oven. Bake for 1 hour, basting the ham once with any remaining glaze.
Remove from the oven and let the ham rest for 5 minutes before slicing. Serve the scalloped potatoes alongside the ham slices.