Preheat oven to 350 degrees. Place half the ham in a buttered casserole dish. Cover with half the potatoes and half the onion. Season with salt and pepper to taste. Sift half the flour over all.
Repeat layering ham, potatoes and onion, seasoning with salt and pepper to taste. Sift remaining flour on top. Pour milk over all.
Cover tightly and bake 1 to 1 1/4 hours. Remove cover. Sprinkle bread crumbs over potatoes. Drizzle with melted butter. Bake another 15 minutes.
OPTIONAL: Grandma would add leftovers like peas or cooked carrots when she had them. Sometimes she would sprinkle shredded cheese on top during the last 15 minutes of baking. Whatever she did it was always good.