Make a few crisscross slits on top of each steak, without cutting them all the way through. Season with 1 tablespoon of salt and black pepper on both sides then set aside.
Chop the onion into large dices, transfer them to a baking dish and evenly spread them all over the bottom. Then add the seasoned steaks, covering the onions, and set aside.
Grate the mozzarella cheese, boil 3 eggs and grate them too. Transfer all these ingredients to a bowl, add 1 level teaspoon of salt and the mayonnaise, mix it all together very well.
Put this mixture over the steaks and spread well again.
Bake in a preheated oven at 200°C (392°F) for about 1 hour.
Serve.