01 - In a large mixing bowl, blend all-purpose flour, instant dry yeast, sugar, and salt. Pour in olive oil and stir. Gradually incorporate warm milk, kneading the mixture until a soft, elastic dough develops, adding extra milk as necessary to achieve the desired consistency. Cover the bowl and allow to rise in a warm location for approximately 60 minutes or until the dough has visibly doubled in volume.
02 - In a separate bowl, combine crumbled feta, shredded mozzarella, finely chopped fresh parsley, nigella or black sesame seeds if desired, egg white, and a generous amount of freshly ground black pepper. Mix until evenly incorporated.
03 - Preheat the oven to 375°F. Line a baking sheet with parchment paper to prevent sticking and ensure an even bake.
04 - Deflate the risen dough and divide into 12 equal portions. Roll each portion into a smooth ball, then flatten into discs approximately 4 to 5 inches in diameter.
05 - Place a generous tablespoon of prepared cheese filling in the center of each dough circle. Fold the edges over the filling, shaping into a triangle or oval, and firmly pinch the seams closed to seal.
06 - Arrange the formed pastries on the prepared baking sheet. Whisk together egg yolk and milk or water, then lightly brush the tops of each pastry with the mixture for a golden finish.
07 - Bake for 18 to 22 minutes or until the pastries turn deep golden brown. Allow to cool slightly before serving.