An onion was an Earth vegetable with a thin, paper-like skin and a structure of tightly-packed concentric rings that were usually segmented during preparation for eating or cooking.
In 1986, Doctor Gillian Taylor ordered a mushroom and pepperoni with extra onions pizza at an Italian restaurant. (Star Trek IV: The Voyage Home)
Based on Dr. Phlox's prior comments, Jonathan Archer surmised that the closest thing Denobulans had to pets were "furry little things that [went] well with onions." (ENT: "A Night in Sickbay")
In 2371, the farmer's daughter's root cellar contained onions. (VOY: "Caretaker")
While operating on Seven of Nine, The Doctor commented that removing her Borg implants was like peeling an onion. (VOY: "The Gift")
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