The Food Lab: Foolproof Onion Rings

It's always difficult to decide whether to get onion rings or fries (get a combo if they'll let you!). Proper beer-battered onion rings, with a substantial crisp crust covering a sweet, tender, thick ring of onion, are one of life's three greatest pleasures (and the only one that can be enjoyed legally, incidentally), but how often do you get perfect rings?