01 - Dissolve sugar in warm milk in a small bowl. Sprinkle yeast over the surface and let stand 5–10 minutes until foamy.
02 - Combine flour and salt in a large bowl. Add melted butter, 1 egg, and the activated yeast mixture. Mix until dough forms.
03 - Knead dough on a lightly floured surface for 7–8 minutes until smooth and elastic. Transfer to a greased bowl, cover, and let rise in a warm area for 1 hour until doubled in size.
04 - Combine chopped hard-boiled eggs, mozzarella, feta, dill, green onions, black pepper, and salt in a medium bowl. Mix until evenly distributed.
05 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
06 - Punch down risen dough and divide into 12 equal portions. Shape each into a ball.
07 - Roll each dough ball into a 4-inch circle. Place 2 tablespoons of filling in the center. Fold dough over filling to create a half-moon shape and pinch edges tightly to seal.
08 - Arrange piroshki seam side down on the prepared baking sheet. Brush each with beaten egg. Bake for 20–25 minutes until golden brown.