Peel and cut onions into 1-centimeter (½ inch) rings. Separate the rings.
Slice the mozzarella into 4 even strips.
Place a smaller onion ring in the center of a larger one.
Fill the gap between the rings with the mozzarella strips. For extra cheesiness, use two layers of cheese. Repeat with remaining onion rings and freeze for 1 hour.
Place the flour, eggs, and breadcrumbs into three separate bowls.
Coat each prepared ring in the flour first. Then dip into the egg, then the breadcrumbs, then the egg again, and one last time into the breadcrumbs. Repeat with the remaining onion rings and place in freezer.
Fill a large pot with oil and set over medium high heat until it reaches 350˚F (175˚C).
Fry the rings until golden brown, being careful not to fry them for too long as the cheese may ooze out completely.