- 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!