Knead the shortcrust pastry together from the butter, flour, egg, salt and the water as needed, and chill wrapped in plastic for 20 minutes.
For the filling, sauté the onion in lard until translucent, add the mushrooms, and braise together for a few minutes.
Sprinkle with flour, add the broth, mix in the leftover meat and the sour cream, season with salt and pepper, then cook down until thick, and let it cool.
Roll out the dough, grease small molds (muffin or pastry tins) with lard, and line them with the dough.
Fill with the cooled hash, cut strips from the remaining dough, and lay them in a lattice pattern on top.
Brush the tops with beaten egg.
Bake in a preheated, moderate oven at 170°C for 25-30 minutes, until golden brown, and serve warm.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.