Prepare the cod for baking. Line a baking tray with foil and grease with vegetable oil. Place the fillet pieces, season with salt and pepper, and coat each piece with mayonnaise or sour cream.
Prepare a cheese and vegetable "fur coat". Cut the onion into thin slices and place on top of the cod. Then place the tomato, cut into thin slices. Grate the cheese on a fine grater and sprinkle on top. Place the baking tray in the oven, preheated to 180°C, bake for 20 minutes until a golden cheese crust forms.