If you choose to use onions (see introduction), peel them, cut them in half, remove the core, and thinly slice them. Sweat the onion slices in a skillet over low heat for 15 minutes. The onion slices should be soft and translucent.
Do not cook the onions over high heat, and turn them regularly. They should not brown.
In a mixing bowl, combine the cream, grated nutmeg, and half the milk. Add the onions, if using. Season with salt and pepper.
Peel the potatoes, rinse them, and slice them thinly (about 2–3 mm).
The Chef’s Tip
You can use a mandoline, which is a very handy tool for slicing thin, evenly thick slices.
Preheat the oven to 170°C. Pour some oil into the bottom of a baking dish. Arrange a layer of potatoes, season with salt, and pour the gratin mixture you prepared earlier over them.
Repeat the same process (potatoes + salt + gratin mixture) for each subsequent layer. For the final layer, pour milk around the edges.
Cover the dish with a sheet of parchment paper or aluminum foil so that the potatoes cook thoroughly without burning on the surface. Place in the oven and bake for 45 minutes.
Reduce the oven temperature to 160°C. Remove the parchment paper, then add the beaten egg yolks, mixed with a little milk, salt, pepper, and nutmeg. Return to the oven and bake for another 45 minutes.