01 - In a small bowl, dissolve sugar in warm milk. Sprinkle yeast over the top and let stand for 5-10 minutes until foamy.
02 - In a large bowl, combine flour and salt. Add melted butter, 1 egg, and the yeast mixture. Mix until a dough forms.
03 - Knead the dough on a lightly floured surface for about 7-8 minutes, until smooth and elastic. Place in a greased bowl, cover, and let rise in a warm place for 1 hour, or until doubled in size.
04 - In a medium bowl, combine chopped hard-boiled eggs, mozzarella, feta, dill, green onions, black pepper, and salt. Mix until evenly combined.
05 - Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
06 - Punch down the risen dough and divide it into 12 equal balls.
07 - Roll each ball into a circle (about 4 inches in diameter). Place 2 tablespoons of filling in the center. Fold the dough over the filling to form a half-moon or oval shape. Pinch the edges tightly to seal.
08 - Place piroshki seam side down on the prepared baking sheet. Brush each with beaten egg. Bake for 20-25 minutes, or until golden brown. Serve warm or at room temperature.