Peel the potatoes and slice them as thinly as possible.

I used a mandoline for this (watch your fingers).

Melt the butter in a pot and pour it into a small jug, without getting the white part. If you have access to a sous vide, it is very easy: Put the butter in a bag and melt it in the water bath. Place the bag at an angle in the fridge or freezer (if you do not have much time).

When the butter has solidified, the whey (the white part) will have settled at the bottom of the bag without solidifying. You can then easily cut a slanted cut at the bottom of the bag to release the whey. Alternatively, if the butter is cold enough, you can take the butter out of the bag without the whey. You now have clarified butter, where the liquid in the butter is gone and it is much better to use for this dish or, for example, Bearnaise.

Add salt, pepper, and thyme to the butter.

Use a rectangular form for the potatoes, such as a bread tin or a cake tin. Brush it with a little melted butter on the sides and bottom (you can also use baking paper if you prefer).

Place a layer of potatoes side by side at the bottom. Brush again with the butter mixture.

Repeat with a new layer and brushing, etc. until all the potatoes and butter are used.

Make sure the top layer is also brushed.

Wrap the whole form in aluminum foil, so the potatoes get a kind of steaming in the butter.

Cook it for an hour at 200 degrees Celsius (392 degrees Fahrenheit) with fan.

Place something very heavy on top of the potatoes so they are pressed together. I used 4x 2 kg of flour.

Refrigerate for 18-24 hours, preferably, and a minimum of 6 hours. So the dish is ideal to make the day before or in the morning.

Take the potatoes out of the form, for example by cutting them into squares of a size you determine yourself. Mine were very tall, so they were actually split in height as well.

When the potatoes are to be served, it is just a matter of getting a little crust and temperature into them - they are already cooked.

This can be done in several different ways:

In a deep fryer at 180 degrees Celsius (356 degrees Fahrenheit), they need about 4-5 minutes.

In a pan, they need 6-8 minutes on each side and you do not need to add fat, there is plenty in the potatoes.

In the oven, they need about 12-18 minutes at 200 degrees Celsius (392 degrees Fahrenheit) with fan.

Bon appétit.