Sweet onions, cut thick, separated into rings by hand, and breaded one batch at a time. It is messy work and your fingers end up wearing half the batter, but a hand-breaded ring holds onto its coating in a way the frozen kind never figured out.
The order matters more than people think. Flour first so the wet batter has something to grab. Then the batter. Then the breading. Skip the flour step and the coating slides right off in the oil, which is exactly how you end up with a naked onion and a sad pile of crumbs.
We use a sweet onion on purpose. When it hits the hot oil that natural sugar softens and turns mellow, so you bite through a crunchy shell into something almost gentle inside. Cook them too long chasing color and the onion turns to mush and slides out the back. There's a window, and finding it by feel is most of the job.