Start by cutting the chicken breast into thin slices, cover them with plastic wrap and pound each slice with a mallet.
Transfer them to a bowl and season with salt and black pepper, add the oil, mix it all together very well.
Chop the onion into strips and transfer them to a baking tin.
Chop the potatoes into round slices and transfer them to the baking tin as well, season them with a generous pinch of salt and black pepper.
Cover the potatoes with all the chicken slices and set aside.
Chop the mushrooms and transfer them to the baking tin as well, season with salt.
Cover the baking tin with aluminum foil and bake in a preheated oven at 200°C (392°F) for 30 to 40 minutes.
Grate the mozzarella cheese and set aside.
Dice the tomato.
Into a bowl, add the cheese, the tomato, the mayonnaise and the egg, mix it all together very well.
After the indicated baking time, take the baking tin out of the oven and spread the mayonnaise mixture all over it.
Bake in a preheated oven at 180°C (356°F) for 15 minutes.
Serve.