Cut the leek whites in half lengthwise to make cleaning easier.

Soak them in a large volume of cold water and shake to remove all the sand. Take them out of the water. Cut them finely.

In a frying pan, cook over medium heat with the butter for 5 minutes.

Add the salt and mix. Gradually pour in the vegetable stock with a ladle until the leeks are well cooked and tender.

Remove from the heat. Set aside.

Remove the half-peel of the lemon with a peeler, or a peeler knife, taking care not to scratch the white skin, which is bitter.

Bring the milk to the boil in a saucepan, remove from the heat, and then add the lemon zest to infuse it for 5 minutes.

Filter the perfumed milk through a strainer.

Melt the butter in a saucepan (be careful that it does not start to color) and then add the flour passed through a dry sieve.

Let it boil slowly for 2 to 3 minutes, stirring constantly, to cook the flour well. With a ladle, add the very hot milk, little by little, taking care to boil the sauce for a few seconds each time you pour in the milk (to avoid lumps). Add a little salt.

Set aside, off the heat.

Preheat the oven to the broil position.

In a salad bowl, mix the leek with the béchamel sauce. Place this mixture in a gratin dish.

Sprinkle with grated Beaufort cheese.

Put in the oven and leave to gratinate for 3 or 4 minutes, more or less depending on the oven, watch carefully!

Take the gratin out and finish the recipe by adding, at the last moment, the grated lemon half-zest.