Briefly blanch the leek rings, drain and simmer gently with the milk over low heat until the liquid is absorbed. Let cool a little and mix with all the other ingredients. If necessary, add salt and pepper.
In a roasting pan you make 6 sheets at the bottom - with oil + butter in between - then you put the mixture on top and another 6 sheets. Brush the surface with a little oil and wet with water.
Bake in the oven for 1 hour at 170 degrees, until the pie turns golden. It can be eaten hot and cold just as well.