Soak the cod in cold water to cover for 24 to 36 hours, changing the water occasionally, drain. Flake the cod into small pieces, removing any bones. Set aside.
In a large saute pan, over medium heat, add 25ml of the oil. When the oil is hot, add the onions and the garlic and season with salt and pepper. Saute until slightly golden for about 6 minutes.
Preheat the oven to 350°C. Grease a medium ovenproof casserole dish with olive oil.
Season the potatoes with salt and pepper. Spread half of the potatoes over the bottom of the prepared dish. Sprinkle half of the salt cod over the potatoes. Place half of the onion mixture over the salt cod. Top the onion mixture with more salt cod. Place another layer of potatoes over the top of the cod. Drizzle the entire pan with the remaining oil.
Place in the oven and bake for 30 to 40 minutes, or until golden.
Cool and serve cold with the chopped hard boiled eggs, olives and chopped parsley.