01 - Combine all-purpose flour, instant dry yeast, sugar, and salt in a large mixing bowl. Drizzle in olive oil and mix until incorporated. Gradually pour in warm milk, kneading until a smooth, elastic dough forms. Add extra milk as necessary. Cover the bowl and let dough rise in a warm environment for 60 minutes, or until doubled in volume.
02 - In a separate bowl, blend together crumbled feta, shredded mozzarella, finely chopped parsley, nigella seeds if desired, egg white, and a pinch of freshly ground black pepper until combined.
03 - Preheat oven to 375°F. Line a baking sheet with parchment paper to prevent sticking.
04 - Gently punch down risen dough and divide into 12 equal portions. Roll each piece into a ball, then flatten into circles approximately 4 to 5 inches in diameter.
05 - Place a generous tablespoon of cheese filling at the center of each dough circle. Fold edges over filling, shaping into triangles or ovals. Pinch seams together firmly for a secure seal.
06 - Transfer shaped pieces to the prepared baking sheet. Whisk egg yolk with milk or water and gently brush mixture over the tops.
07 - Bake in preheated oven for 18 to 22 minutes, or until pastry is golden and crisp. Allow to cool slightly on a rack before serving.