Cut the salt cod into 8-10 serving pieces and soak them in water for 24 hours, changing the water 3-4 times. Discard any bones. Place the salt cod in a large saucepan and cover with water. Bring to a boil, reduce heat, and simmer for 5 minutes. Boil the potatoes for about 20 minutes. Melt the butter in a large flameproof casserole and cook the onions until softened, about 5 minutes. Sprinkle the flour into the onions and cook, stirring once or twice, for 2 minutes. Drain the cod, reserving the cooking liquid. Stir 1/2 cup of the cooking liquid into the onions, along with the cream, and salt cod. Season with pepper and bring to a boil. To serve, thinly slice the potatoes and place them around the edge of a serving platter. Pour the cod mixture into the center of the platter and sprinkle with the parsley.
