Slice onion into rings about ½ inch wide. Slice fresh mozzarella cheese into long slices, ½" wide. Pair onion rings up so that slices of cheese and fit tightly between the slices. Push the cheese between the onion rings, filling in the gap completely. dredge the onion cheese rings with flour and place in freezer for at least 45 minutes, overnight is best. Dip the frozen rings into the beaten egg, then coat completely with breadcrumbs. Fry in peanut or canola oil until lightly golden brown. Drain on paper towels and serve hot with a side of marinara sauce.