Soak the dried cod in cold water for about 24 hours, or until completely moistened. Change the water several times; drain thoroughly.
Put the cod into a saucepan with cold water to cover. Bring to a boil, reduce heat, and simmer for 15 minutes or until fish is tender. Drain; skin and bone the fish. Flake with a fork into large pieces.
Saute the onions in half the butter until they are tender and golden; Add the garlic. Boil the unpeeled potatoes in salted water. Once tender, remove from the heat, refresh in running cold water; then remove skins.
Drain and slice into 1/4" pieces.
Preheat oven to 350°F Grease a 1-1/2 quart casserole with the remaining butter. Arrange a layer of half the potatoes, then half the cod, then half the onions. Sprinkle with a little pepper, and repeat layering. Sprinkle bread crumbs over the top layer. Bake for 15 minutes, or until heated through and lightly browned.