01 - In a large mixing bowl, combine all-purpose flour, instant dry yeast, sugar, and salt. Add olive oil and mix to incorporate. Gradually pour in the warm milk, mixing and kneading by hand until a soft, elastic dough forms. Add a splash more milk if the dough feels dry. Cover the bowl with a clean towel and allow the dough to rise in a warm environment for approximately 60 minutes, or until doubled in volume.
02 - In a separate bowl, blend crumbled feta, shredded mozzarella, chopped parsley, nigella or black sesame seeds if using, and egg white. Season lightly with black pepper, stirring to combine into a cohesive filling.
03 - Preheat the oven to 375°F. Line a baking sheet with parchment paper to prevent sticking.
04 - Punch down the risen dough and divide into 12 equal portions. Roll each piece into a ball, then flatten into a disc approximately 4 to 5 inches in diameter.
05 - Place a generous tablespoon of the cheese mixture in the center of each dough round. Fold edges over the filling to shape into triangles or ovals, firmly pinching seams to seal.
06 - Transfer assembled pastries onto the prepared baking sheet. In a small bowl, whisk the egg yolk with milk or water and brush the tops evenly to promote browning.
07 - Bake pastries on the center rack for 18 to 22 minutes, or until golden brown. Remove and cool slightly before serving.