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 an oven to 180 degrees C.
Place the cod in a saucepan with enough water to cover. Bring to a boil and cook for 10 minutes. Remove the cod from the saucepan then remove and discard the skin and bones. Cut the cod into chunks and set aside.
Heat 2 tablespoons olive oil in a frypan then cook the potatoes in the hot oil until just cooked; about 5 minutes. Transfer to a plate lined with paper towels to drain.
Pour 2 tablespoons more olive oil to the frypan then cook the onion and garlic until the onion is translucent; about 5 minutes.
Add the cod to the frypan then cook another 3 minutes before stirring the potatoes into the mixture; cook another 1 to 2 minutes. Reduce heat to low.
Melt the butter in a separate frypan over medium-low heat then 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 frypan with 1 cup thickened cream stirring to coat. Allow the mixture to simmer together for about 2 minutes. Season with salt and pepper then transfer to a small baking dish.
Drizzle another 1/2 cup of thickened cream over the mixture then sprinkle with the Parmesan cheese.
Bake in the preheated oven until the top is browned; 30 to 40 minutes. Serve hot.