Layer the ham and potatoes in shallow 1 1/2 quart baking dish. Melt the butter in sauce pan. Add onion and cook till golden. Blend in flour; add milk gradually and cook, stirring constantly till thickened. Season with salt & pepper. Pour over ham/potato mix and sprinkle with cheese. Dot with remaining butter. Bake at 400 for 20-25 min. Serves 4 generously.