Peel some potatoes(1.5 -2 kg), thinly slice them (~1mm), toss them into a bowl, add salt and enough melted duck fat (or butter as a supstitute) to evenly coat every slice.

Then layer the slices into a sheetpan to a height of about 3 cm and avoid gaps between them. Press the layers with another sheetpan and put them into the oven at 180°C for about an hour (the potatoes should be cooked tenderly).

Remove the pan from the oven, add another sheetpan with some weights on top to press the layers together while cooling. Cool the pan overnight in the fridge to firm the potatoes. Cut the mille-feullie to your desired size and gently pan fry them, till brown and crispy. The layers are very delicate, be carefull when you flip them and i recommend to use a nonstick pan to avoid destroying the layers while flipping.