Pike perch clean, remove bones and disassemble on filet. Put together with meat and onions in a blender and chop, you can grind in a meat grinder.
Peel and chop the potatoes, add to the minced meat, season with salt and pepper, mix well.
Roll out the dough, put the filling in the center and blind the oblong cake with a seam on top, leave a hole in the middle of the cake. Brush with a beaten egg and place in a cold oven, then turn on 170 degrees and bake until golden brown, about 30-45 minutes.
Let kulebyaki cool a bit and serve.