Prepare the fish. Cut the onion into half rings. Grate the cheese on a coarse grater. Cut the pollock fillet into pieces and sprinkle with lemon juice. Add pepper and salt to taste.
Prepare the sauce. Place mayonnaise, mustard and sour cream in a separate bowl. Mix.
Bake the fish. Preheat the oven to 180 degrees. Grease the baking dish with vegetable oil. Put the chopped onion as the first layer. Dip one side of each fillet piece in the sauce and place it on the onion with this side. Pour the rest of the sauce over the fish in the dish. Sprinkle the dish with grated cheese. Bake the fish for 20 minutes.