Cut the onions into 1/2-inch thick slices and separate them into rings. Find pairs of similar-sized rings and set aside.

Cut the mozzarella sticks to fit the height of the onion rings.

Place a smaller onion ring inside a larger one, leaving a gap between them. Insert a piece of mozzarella stick in the gap between the onion rings. Repeat with the remaining onion rings and mozzarella sticks.

Season the flour with salt and pepper. Dredge each stuffed onion ring in the seasoned flour, shaking off the excess.

Dip the floured rings into the beaten eggs, then coat them in breadcrumbs, making sure they are completely covered. For extra crunch, you can double-bread by dipping them in egg and breadcrumbs again.

Place the breaded onion rings on a baking sheet lined with parchment paper. Freeze for at least 1 hour to firm up, which helps prevent the cheese from oozing out during frying.

Heat oil in a deep fryer or large pot to 350°F (175°C). Fry the onion rings in batches, being careful not to overcrowd the fryer, until golden brown and crispy, about 2-3 minutes per batch.

Remove with a slotted spoon and drain on paper towels.

Serve the Mozzarella Stick Onion Rings hot with marinara sauce, ranch dressing, or your favorite dipping sauce.