An onion crown on a watch typically refers to an oversize crown on flieger style watches.
Onions crowns look cool, but they serve a practical purpose: for a pilot to be able to wind the crown and set the time while wearing gloves in flight.
Below is one of the original onion crown wristwatches from 1910.
The traditional onion crown is large and rounded with etched lines that allow for a good grip. Probably the most recognizable example with this style of onion crown would be the Chronoswiss Timemaster.
Another style of onion crown is more flat, with a popular example being found on the Zenith Pilot Type 20.