Put the butter, flour, and salt in a food processor and process until the mixture resembles fine crumbs.
Transfer to a bowl and add the sour cream and egg. Stir until everything is combined and you get the dough. Form to a ball, wrap in plastic wrap and refrigerate for 1 hour.
Meanwhile, hard boil the eggs for the filling and chop them. Thinly slice the cabbage and add some salt to taste.
In a large pan, heat the oil and butter and cook the cabbage for 3-5 minutes. Put a lid on and cook for 10 minutes or more until it’s soft. The cooking time depends on the toughness of the cabbage. Let cool and combine with the hard-boiled eggs.
Preheat the oven to 350F (175C). Lightly grease the round 9-inch baking pan (I used a tart pan) with butter.
Divide the dough into two parts: one slightly larger than the other. Roll out the larger part on a lightly floured surface and line the pan. Trim the excess dough from the edges. Roll out the smaller dough part and cut into even strips.
Add the cabbage and egg filling to the dough-lined pan and top with the dough strips to form a lattice. Gently press both ends of the strips into crust edges to keep their shape when baking.
Lightly beat the egg yolk with a small pinch of salt and brush the strips. Bake 40-45 minutes. Enjoy!