Boil the spinach for 2 minutes in salted water. If using frozen and lightly pre-cooked spinach, place it in hot water for 5 minutes.
Carefully dry the spinach with kitchen paper and chop it into small pieces.
Wash each piece of chicken, dry it with kitchen paper, and make a small incision.
Cut the mozzarella ball into thin slices and stuff the fillet with mozzarella and spinach.
Break the egg into a bowl and beat it. Dip the fillets first in the egg and then in the breadcrumbs and place them on a baking tray.
Bake at 175°C (347°F) for 25 minutes.