Rinse the fish and place it in a bowl. Add enough cold water to cover. Chill overnight, changing the water several times.
The next day, drain the fish and transfer to a large saucepan. Cover with water, bring to a boil, and simmer until the fish flakes easily, about 15 minutes.Drain and cool. Flake the fish, discarding any bones.
Heat 4 tablespoons of the oil in a heavy, large nonstick skillet over medium-high heat. Add the potatoes in batches and sauté until crisp and golden, about 7 minutes per batch. Transfer the potatoes to paper towels to drain.
Add 1 tablespoon of the oil to the same skillet. Add the onion and and sauté until golden, about 15 minutes. Reduce the heat to low. Add the remaining 2 tablespoons oil to the onion slices in the skillet.
Mix in the fish and potatoes. Whisk the eggs, the 1/2 teaspoon salt, and the 1/2 teaspoon pepper in a large bowl to blend. Add the egg mixture the skillet.
Cook over medium heat until the eggs are softly set, stirring occasionally, about 3 minutes. Transfer onto a platter.
Garnish with the olives and the parsley.And serve with some salad.
