Wash the potatoes, add cold water, and bring to a boil. Boil until tender in a uniform over low heat, covered (about 20 minutes). Remove the potatoes, cool.
Peel the potatoes, cut into circles.
Turn on the oven. Peel, wash, chop the onion.
Heat a frying pan, pour in vegetable oil. Put the onion in hot oil. Lightly fry the onion over medium heat, stirring occasionally (1-2 minutes).
Grate the cheese on a coarse grater.
Cut the fish into pieces, salt, and pepper.
Take half of the onion. Put pieces of fish in a pan with the remaining fried onions. Then on top – the second part of the fried onion.
Then put circles of boiled potatoes around the fish.
Pour all over with mayonnaise.
Then sprinkle with grated cheese.
Place in the oven on the middle shelf. Bake the fish in the oven until cooked at a temperature of 190 degrees (40-45 minutes).