01 - In a large mixing bowl, blend the flour, instant dry yeast, sugar, and salt. Incorporate olive oil and gradually add warm milk, kneading until a soft, elastic dough forms. Add additional milk if necessary. Cover the bowl with a clean cloth and allow the dough to rise in a warm area for 60 minutes until doubled in volume.
02 - In a separate bowl, combine crumbled feta, shredded mozzarella, chopped parsley, nigella or black sesame seeds if using, egg white, and freshly ground black pepper. Mix thoroughly to ensure even distribution.
03 - Preheat oven to 375°F. Line a baking sheet with parchment paper to prevent sticking.
04 - Punch down the risen dough. Divide into 12 uniform portions, rolling each into a ball. Flatten each dough ball into a 4- to 5-inch circle. Place a generous tablespoon of cheese filling in the center.
05 - Fold dough edges over the cheese filling, forming triangular or oval pastries. Pinch the seams firmly to ensure each pocket is well sealed. Arrange pastries on the prepared baking sheet.
06 - Whisk together egg yolk and milk or water. Brush the surface of each fatayer with the mixture. Bake in the preheated oven for 18 to 22 minutes, or until the pastries turn golden brown.
07 - Allow the fatayer to cool slightly on a rack before serving warm or at room temperature.