01 - In a large mixing bowl, combine the all-purpose flour, instant yeast, sugar, and salt. Drizzle in the olive oil and stir gently. Gradually add the warm milk while kneading until a supple and elastic dough forms, adding additional milk if required. Cover with a clean towel and allow to rise in a warm setting for 60 minutes or until doubled in size.
02 - In a separate bowl, combine crumbled feta, shredded mozzarella, finely chopped parsley, nigella seeds if using, egg white, and freshly ground black pepper. Blend until uniformly mixed.
03 - Preheat oven to 375°F. Line a baking sheet with parchment paper for even baking and easy release.
04 - Gently deflate the risen dough and split into 12 equal portions. Roll each piece into a ball, then flatten into a round approximately 4 to 5 inches in diameter.
05 - Place a generous tablespoon of cheese filling in the center of each dough circle. Fold dough over the filling to form triangle or oval shapes, pinching the seams to seal securely.
06 - Transfer shaped pastries to the prepared baking sheet. Whisk together the egg yolk with milk or water and gently brush the tops for a rich, golden finish.
07 - Bake in the preheated oven for 18 to 22 minutes, or until golden brown. Let pastries cool slightly before serving warm.