Boil the potatoes until half cooked in salted water.
Finely chop the onion and fry in butter.
Grate the cheese on a coarse grater.
Pour flour into the cold milk, add a pinch of nutmeg, mix everything thoroughly.
Add the cheese, setting aside the third part for sprinkling. Put on the fire and, stirring slightly, heat up, bring to a thickening.
In a baking dish, put the potatoes cut into thin slices, pour oil with onions. Keep alternating layers.
Pour cheese sauce on top. Place in a preheated oven for 30 minutes.
Mix the delayed cheese with breadcrumbs. Remove the baking sheet from the oven, sprinkle with the resulting mixture. Put in the oven for another 30 minutes.
Enjoy your meal!