01 - In a large mixing bowl, combine all-purpose flour, instant yeast, sugar, and salt. Add olive oil and mix thoroughly. Gradually incorporate warm milk, kneading until a soft, elastic dough forms. If necessary, add extra milk, a tablespoon at a time, to achieve desired texture. Cover bowl with plastic wrap or a clean towel and set aside in a warm location for 60 minutes until dough doubles in volume.
02 - In a separate bowl, blend crumbled feta, shredded mozzarella, chopped parsley, nigella or black sesame seeds if using, egg white, and a generous crack of black pepper. Mix until well combined and set aside.
03 - Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper for easy removal and even baking.
04 - Deflate the risen dough and divide evenly into 12 portions. Roll each piece into a smooth ball. On a lightly floured surface, flatten each ball into a round measuring 4 to 5 inches in diameter.
05 - Place a generous tablespoonful of cheese mixture at the center of each dough round. Fold sides over to create a triangle or oval shape, firmly pinching seams to securely enclose the filling.
06 - Transfer shaped pastries onto the prepared baking sheet. Whisk egg yolk with milk or water and brush over the tops for an appealing sheen and color.
07 - Bake in the preheated oven for 18 to 22 minutes, or until golden brown and puffed. Allow to cool slightly before transferring to a serving dish.