Prepare a standard enriched yeast dough using the flour, milk, sour cream, oil, sugar, salt, yeast, and egg. Knead until smooth and elastic.
Cover and leave in a warm place until well risen and approximately doubled in volume.
Meanwhile, prepare the filling. Finely slice or shred the cabbage and sprinkle it with the salt.
Place the salted cabbage in a large bowl and pound or firmly massage it until it begins to release its juices.
Finely chop the onion. Melt part of the butter in a large pan and sauté the onion until softened.
Add the cabbage together with the liquid it has released. Cover and gently braise over low heat until tender.
Gradually add the remaining butter while the cabbage cooks. Season with the sugar and a small amount of citric acid.
Allow the cabbage mixture to cool.
Peel and finely chop the hard-boiled eggs, then fold them into the cooled cabbage filling.
Once the dough has risen, roll it approximately 1 cm (⅜ inch) thick and divide it into two unequal portions.
Place the smaller piece of dough onto a greased baking sheet.
Spread the prepared cabbage filling evenly over the dough, leaving a border around the edges.
Cover with the larger piece of dough. Fold its edges underneath or around the edges of the bottom layer to enclose the filling.
Leave the assembled pie to rise again briefly.
Brush the top with beaten egg and prick it in several places with a fork to allow steam to escape.
Bake in a moderately hot oven until the dough is cooked through and deeply golden, approximately 35–45 minutes at 180°C / 350°F.
Allow to cool slightly before cutting into square pieces, as instructed in the original cookbook.