In a large bowl, combine the flour and salt.
Cut in the cold butter until the mixture resembles coarse crumbs.
Gradually add cold water, mixing until the dough comes together. Do not overwork the dough.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Heat the vegetable oil in a pan over medium heat.
Add the chopped onion and cook until soft and translucent, about 5 minutes.
Add the ground pork or beef and cook until browned. Drain any excess fat.
Add the diced potato, black pepper, paprika, and salt. Cook for an additional 10 minutes, or until the potatoes are tender. Let the filling cool slightly.
Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Roll out the chilled dough on a floured surface to about 1/8 inch thickness.
Cut the dough into circles (about 4-5 inches in diameter).
Place a spoonful of the filling in the center of each dough circle.
Fold the dough over the filling to form a half-moon shape and pinch the edges to seal. You can also crimp the edges with a fork.
Place the filled pirukas on the prepared baking sheet.
Brush the tops with beaten egg.
Bake in the preheated oven for 20-25 minutes, or until the pirukas are golden brown.
Enjoy your meal!