Skin the cod using large scissors. Cut and remove the fins, large bones and any parts other that the main piece. Divide the cod into smaller pieces and place in a large bowl. Cover with the milk, and leave to desalt for 24 hours. Drain and dry the fish, put it in a large casserole, cover with cold water and heat, bringing it slowly to a boil. As soon as it boils, drain and flake the cod. Chop the onions, brown them in 2 tablespoons of butter. Peel and slice the potatoes, cut the Cantal cheese into strips. Butter an ovenproof casserole. Place succcessively a layer of cod, a layer of potatoes, a layer of Cantal and the onions. Finish with a layer of potatoes. Sprinkle small amounts of butter over the top, add pepper and bread crumbs. Bake at 410F for 30 minutes.
