The usual filling is feta cheese; this version calls for Gouda and Emmenthal. If you have never worked with filo pastry before, know that it is very fragile and must be kept covered or it will dry out very quickly. This is labour-intensive, but worth it. I like to divide the filling ahead of time into 48 equal little portions to ensure that all the borek are uniform in size. This may be made a day before they are baked, and stored covered in the refrigerator.