01 - In a large mixing bowl, combine all-purpose flour, instant dry yeast, sugar, and salt. Add olive oil and mix well. Gradually pour in the warm milk, kneading until a soft, elastic dough forms. If required, add an extra splash of milk for the correct consistency. Cover and let rest in a warm location for about 60 minutes, until the dough has doubled in volume.
02 - In a separate bowl, blend together crumbled feta, shredded mozzarella, chopped parsley, nigella or black sesame seeds (if using), egg white, and a pinch of black pepper to create a uniform cheese mixture.
03 - Set oven to 375 degrees Fahrenheit. Line a baking sheet with parchment paper for easy removal and minimal sticking.
04 - Deflate the dough and portion into 12 equal pieces. Roll each into a ball, then flatten with a rolling pin into circles approximately 4–5 inches wide.
05 - Place a generous tablespoon of the cheese filling in the center of each dough circle. Fold the edges over the filling to form a tight triangle or oval, pinching seams firmly to prevent leaks.
06 - Transfer shaped pastries onto the parchment-lined baking sheet. Whisk egg yolk with milk or water, and use a pastry brush to coat the tops for a glossy finish.
07 - Bake for 18 to 22 minutes, or until the surface is deeply golden and crisp. Allow to cool slightly before serving.