1. Place the eggs in a pot, fill the pot with cold water to cover the eggs. Bring the water to boil, over high heat. Reduce the heat and boil the eggs for 10 minutes to get hard boiled eggs.
2. Meanwhile prepare the other ingredients. Chop the onion, grate the cheese and butter.
3. When the eggs are ready, chill them by placing under cold running water for a few minutes. Then peel the eggs and separate whites from yolks. Grate whites and yolks separately.
4. Drain oil from fish, remove bones and skin (if there is any), and mash the fish with the fork.
5. Organize the ingredients into layers (in a salad bowl with straight sides) in the following order:
3) 1/2 of the mashed fish
4) put a thin layer of mayonnaise to cover the fish
7) the rest of fish
8) thin layer of mayonnaise
6. 9) decorate the top with yolks
7. Let cool in refrigerator for at least 1 hour. Use a knife to cut into segments. Serve.