Two days before making, soak the cod in 1 quart of the milk in a covered bowl in the refrigerator for 24 hours. The next day, discard the milk and rinse the cod well with cold water. Put the cod in a fresh quart of milk and refrigerate for another 24 hours. Discard the milk and rinse well with cold water. Pat dry and flake into small pieces. Set aside.
In a large, nonstick saute pan, over medium heat, add the oil. When the oil is hot, add the onions. Season with salt and pepper. Saute for 4 to 6 minutes, or until the onions are wilted. Add the potatoes and the cod. Season with black pepper. Continue to saute for 4 minutes. Season the eggs with salt and pepper. Add the egg mixture and stir constantly to scramble the eggs. Cook until the eggs are soft, but slightly firm, about 4 minutes. Remove from the heat and spoon into serving plates. Garnish with sliced olives.