Clean capelin, removing the head, tail and visceras. Slice onions into semi-circles and cut potatoes into very thin plates. Salt and pepper all the "filling" ingredients to taste (you can flavor the mixture with special seasonings for potatoes or seafood). Put the potatoes first, then the fish, and then the onion on the oiled sheet-pan. Knead dough just before placing the pie in the oven. To do this, combine milk, flour, mayonnaise and eggs, add half tablespoons salt and sugar and stir all the ingredients thoroughly. Pour the dough evenly over the filling laid on the sheet-pan and slightly shake the pan, so that the dough covers all the fish and potatoes. Place the pie in the preheated oven for about 40 minutes. Keep watch over the pie: when it is browned enough, you can take it out and serve.