01 - In a large bowl, combine flour, instant dry yeast, sugar, and salt. Add olive oil and mix thoroughly. Gradually incorporate warm milk, kneading to achieve a soft and elastic texture. Add additional milk if the dough appears dry. Cover and allow to rise in a warm location for 60 minutes or until doubled in volume.
02 - In a separate bowl, blend crumbled feta, shredded mozzarella, finely chopped parsley, nigella seeds if using, egg white, and a touch of freshly ground black pepper until evenly combined.
03 - Set oven to 375°F. Line a baking sheet with parchment paper to prevent sticking.
04 - Punch down the risen dough and divide into 12 even portions. Roll each into a ball, then flatten to circles approximately 4 to 5 inches in diameter.
05 - Place a generous tablespoon of cheese mixture in the center of each dough circle. Fold the edges to enclose the filling and shape into a triangle or oval, pinching firmly to seal seams.
06 - Place shaped pastries onto the prepared baking sheet. Whisk egg yolk with 1 tablespoon milk or water and brush evenly over the tops. Bake for 18 to 22 minutes until golden brown. Cool slightly before serving.