Large fully cooked ham
In a medium saucepan, sauté onion in butter. Stir in arrowroot powder and salt. Pour in milk and cook over low heat, stirring constantly until bubbly. Remove from heat.
Scrub potatoes and slice very thin. Place ham in the middle of a large baking pan. Lay one half of the potatoes around the ham. Pour one half of the cream sauce over the potatoes. Repeat layers, making sure the potatoes are covered completely with sauce.
Cover and bake at 325 degrees for 1 hour. Uncover and bake another 15 minutes.