1 tbsp sesame or poppy seeds for topping (optional)
Nonstick cooking oil spray
Preheat oven to 350 F. In a mixing bowl, combine feta,mozzarella , egg, a pinch of salt and a pinch of black pepper (if using all feta cheese, you may not need to add any salt). Use a fork to mix ingredients together till well blended. Make sure to break up any large crumbles of feta with the fork. Reserve mixture.
On a smooth, clean, lightly floured surface, unfold one of your puff pastry sheets. Use a rolling pin to roll out the sheet to a 12x12 inch square. If using homemade puff pastry, roll your dough out to the same size - a 12”x12” square. Cut the sheet of puff pastry dough into 9 equal-sized squares, each about 4”x4” large.
Place 1 scant tablespoon of the cheese filling in the center of each dough square.
Fold the dough squares by grasping one corner and folding it over to the opposite corner to make triangles. Pinch firmly along the outer open edge of the triangles to seal. If you're having trouble sealing the dough and getting it to stick together, wet your finger with a bit of water and run it around the edge of the square before folding.
You can also crimp the edges with the tines of a fork, if you wish.
Repeat this process for the second sheet of puff pastry—roll out the pastry, cut into squares, add filling, and seal the triangles
Spray your baking sheets with nonstick cooking spray or line with parchment paper. Place 9 bourekas on each sheet, evenly spaced, giving them some room to expand during baking.
In a small bowl, whisk together the egg yolk and 2 tsp of cool water. Use a pastry brush to brush a light layer of the egg wash onto the surface of each boureka.
Sprinkle the bourekas with sesame or poppy seeds, if desired.
Bake the bourekas for about 30 minutes, switching the baking sheets between the upper and lower racks halfway through cooking. Bake till golden brown and cooked through.
Serve warm. Store in a sealed container or plastic zipper bag.