Where this dish comes from
Mozzarella sticks trace their American restaurant history to the 1970s, when they appeared on chain restaurant menus and quickly became one of the most popular appetizers in American food service history. The preparation is a variation on the ancient technique of frying cheese — found across Italian, Spanish, and Eastern European cooking — adapted for American bar food culture. The key insight that makes them work — freezing before frying — is the kind of empirically discovered restaurant technique that doesn't appear in cookbooks but gets passed from line cook to line cook.
Cheffy’s note
Freeze them. This is not optional advice. If you fry unfrozen mozzarella sticks, the cheese melts faster than the crust sets and you get a puddle of cheese with a loose breading shell sitting in the oil. The freeze is what makes this technically possible. Use low-moisture mozzarella, not fresh — fresh mozzarella is too wet and will explode.