1Peel potatoes and cut into thin slices.
2In a pan, heat plenty of olive oil and sauté potatoes.
3Drain potatoes on paper towel and set aside.
4Peel onion and cut into julienne strips and sauté in olive oil.
5When onion is cooked, shred boneless, skinless cod and add to pan.
6Sauté briefly and set aside.
7For the cream sauce, melt butter in a small skillet over low heat, add flour and fry briefly, then whisk in milk, stirring constantly to avoid lumps.
8Tip: If sauce is lumpy, smooth out in the blender.
9Season with salt, pepper and nutmeg.
10Simmer over low heat until sauce thickens, stirring constantly.
11In a baking dish, arrange a layer of potatoes in the bottom of dish, then add a layer of cod and onion.
12For the final layer, evenly spread white sauce. If desired, add chile pepper to the layer of cod loin and onion.
13For best results, cover and refrigerate overnight.
14Before serving, bake at 200°C (approximately 400°F) for about 10-15 minutes.