Wash the potatoes well, place them in a pan, cover them with plenty of cold water and bring to the boil, leaving them to boil for 25 minutes or until the potatoes are quite soft (without falling apart).
While the potatoes are cooking, clean the cabbage, removing even the double ribs from the leaves, then rinse it well and boil it in lightly salted boiling water for about 5 minutes, then drain.
Then move on to the béchamel sauce: heat the milk until it almost reaches the boil; separately, let the butter melt in a saucepan over a low heat, incorporate the flour all at once and mix immediately; let it cook for 4-5 minutes, stirring constantly, then remove from the heat and slowly add the hot milk, continuing to mix, finally put it back on the heat and let it thicken, always over a low heat and always stirring.
Finally remove from the heat and season with salt and nutmeg.
When the potatoes are well cooked, drain and peel them, then let them cool at least before cutting them into slices about 0.5-1 cm thick.
Start assembling the dish: pour a little béchamel on the bottom of a baking dish suitable for cooking in the oven, add a layer of cabbage, a layer of potatoes, more béchamel and grated Parmesan.
Continue like this until all the ingredients are used up and conclude with cabbage and parmesan.
Complete with a drizzle of oil and cook for about 20 minutes in a preheated fan oven at 180°C, then turn on the grill and leave to cook au gratin for 5 minutes or until golden.
The black cabbage flan is ready, let it rest for 10 minutes before serving.
