Soak the dried salted cod in cold water with the skin side up for 24 hours, changing the water about 4 times. Drain and discard the water.
Preheat oven to 180 C / Gas 4.
Place the cod in a pan with enough water to cover; bring to the boil and cook for 10 minutes. Remove the cod from the pan; remove and discard the skin and bones. Cut the cod into chunks and set aside.
Heat 2 tablespoons olive oil in a frying pan; cook the potatoes in the hot oil until just cooked, about 5 minutes. Transfer to a plate lined with kitchen towels to drain.
Pour 2 tablespoons more olive oil to the frying pan; cook the onion and garlic in the frying pan until the onion is translucent, about 5 minutes. Add the cod and cook another 3 minutes; stir the potatoes into the mixture and cook another 1 to 2 minutes. Reduce heat to low.
Melt the butter in a separate frying pan over medium-low heat; vigorously stir the flour into the melted butter. Slowly stream the hot milk into the mixture while stirring; cook and stir until thick. Season with the nutmeg. Pour into the frying pan with 125ml double cream; stir to coat. Allow the mixture to simmer together for about 2 minutes. Season with salt and pepper; transfer to a small baking dish. Drizzle another 125ml double cream over the mixture; sprinkle with the Parmesan cheese.
Bake in the preheated oven until the top is browned, 30 to 40 minutes. Serve hot.
Can be found in Mediterranean or Carribean speciality shops.
Alternatives to cod:
Try this recipe with inexpensive fresh coley or pouting. They both belong to the cod family but are much cheaper and available in larger supermarkets or fishmongers.