Peel and dice the potatoes, trying to have pieces that are approximately the same size.
Cook them. If you use an Instant Pot: cook at high pressure for 6 minutes. If you use your stovetop, cook the potated in salted water until their tender. Do not overcook them, as they will later go in the oven.
Béchamel sauce (aka white sauce)
In a saucepan, melt the butter.
Then, add the flour. Whisk it until there is no clumps. Cook the roux for 1 to 2 minutes, to avoir the taste of raw flour.
Gradually add the milk (do not dump it all at once!). Whisk well in-between each addition. The sauce should be more liquid than usual.
Season with salt, pepper, and nutmeg once the desired consistency is reached.
Putting it together
Preheat your oven to 350°F (= 180°C = thermostat 6).
In a gratin dish, add a layer of potatoes. You can season them however you like; we used some spice powder mix. Add a few pieces of cod. As for the potatoes, try to have fish pieces equal in size for better cooking.
Repeat until all the potatoes and fish are used.
Pour the béchamel sauce. That's why it should be more liquid than usual: so it goes between all the food.
Add some grated cheese on top.
Cook for about 20mn.
In the video, we use a premix product from Maïzena to make the béchamel. It's a very good alternative when you are in a hurry.You can season the potatoes before putting the dish together. Use any spice that you like, plus salt.