I had a package of frozen spinach and left over fresh spinach. I boiled a half cup of water, added frozen spinach, some vegetable oil, covered it up, until unfrozen. Then turned the heat off and added fresh spinach covered them up until they got tender. (or instead , you use 2 package of frozen spinach) !In a small pot, I heated 2 spoonfuls of oil, chopped dry onions and sautéed until onions got softer. Turned it off, cooled and added feta, salt and pepper. Mix this cheese and spinach in a bowl, keep it ready for filling. At this point you can boil potatoes if you want to make Böreks with potato too!