Peel an onion and cut it into round slices, about 1-finger thick each.
Release the onion rings and remove the core.
Cut the mozzarella block into small strips, make sure they're not too thick, or they won't fit into the onion rings.
Place a smaller onion ring inside a larger one and fill the gap between them with the mozzarella strips.
Repeat this process until you've used up all the onion rings and cheese strips.
Into a bowl, crack the eggs and whisk them.
To make the breading, start by coating the onion rings in the wheat flour, then dip them in the eggs and finally coat them in the Panko flour. If you don't have Panko flour, you can use breadcrumbs instead.
Fry in very hot oil.