Slice the onions into 1 cm thick rings. To make the filling easier, only pair up every second ring.
Attach the thin slices of cheese and ham (or bacon) inside the bigger onion rings.
Then put the other onion rings in their place.
Coat the filled onion rings in the flour, then in the egg yolk, and finally in the breadcrumbs.
The only thing left to do is to fry them in hot oil, then you can serve them.
Good luck with the preparation, and have a nice meal!