Preheat the oven to 375°F (190°C).
Boil the potatoes in salted water for about 10 minutes, or until tender. Drain and set aside.
Meanwhile, in a large skillet, sauté the onion in butter over medium heat until soft and translucent.
Add the flour to the skillet and stir until well combined.
Slowly pour in the milk, whisking constantly, until the mixture is smooth.
Add the salt, black pepper, and nutmeg to the skillet and stir well.
Add the fish fillets to the skillet and cook for about 5 minutes, or until the fish is cooked through and flakes easily with a fork.
Add the cooked potatoes to the skillet and stir gently to combine with the fish and sauce.
Transfer the mixture to a baking dish and bake for 10-15 minutes, or until the top is lightly browned and crispy.
Sprinkle the chopped parsley over the top of the Plokkfiskur and serve hot with a side of dark rye bread.
