Wash the mackerel and dry with paper towels. Defrost the dough. Rinse the vegetables. Separate the yolk from the white. Preheat the oven.
Clean the mackerel from entrails and films and cut into boneless fillets. Cut the onion into thin half rings, grate the carrots on a fine grater.
In a preheated frying pan with butter, fry the onions and carrots until golden brown.
On a floured board, lightly roll out the dough and cut into 4 equal layers. Beat the yolk.
Place a small amount of fried vegetables in the middle of each layer, with half a fish fillet on top. To the right and left of the fish, make longitudinal, frequent cuts and weave a braid out of them so that the fish is inside the dough. Brush the top with yolk.
Transfer the dough with fish to a baking tray and bake in an oven preheated to 180°C for 30 minutes. Serve the fish pie warm.
Serve the pie with fish soup or fish soup, or as a separate dish, garnished with fresh herbs.