Braise the leeks in a little water. Discard the water and replace with milk, until the leeks are soft and have absorbed the liquid. Leave the leeks to cool, then add the feta and the eggs, beaten. Add a little pepper, and salt, if you like.
Layer half the pastry sheets in a rectangular baking tin, brushing olive oil and adding kefalograviera on top of each sheet. Add the pie filling and continue with the rest of the pastry sheets. Cut lines into the top of the pastry, and brush with the egg. Bake at 250° C for 1 hour until golden.