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Fill a medium-sized pan with water and a good pinch of salt and bring to a boil. Rinse the potatoes and cut them into thick slices. There's no need to peel the potatoes.
Boil the potatoes al dente in about 10 minutes. They don't need to be fully cooked yet as they continue to cook in the oven.
Preheat the oven to 180 degrees Celsius and coat a large oven dish with butter.
In the meantime, remove the hard parts of the endive and cut the endives into long strips.
Heat a knob of butter in a large skillet and fry the endives for a few minutes, making sure that as much moisture as possible evaporates. If it still contains too much moisture, the gratin will become too soggy.
Season the endives with salt and pepper. Divide over the bottom of an oven dish and place the potato slices on top.
Mix eggs with heavy cream, crème fraîche, a pinch of nutmeg, pepper and salt. Gently pour the mixture over the potatoes and endives. Divide a handful of grated cheese over the top of the casserole.
Bake the gratin until golden brown in about half an hour. Garnish with fresh parsley if you like, enjoy!