The Chinese word for onion is
洋葱

yáng cōng

Chinese Definition

Components
洋
1. multitudinous; vast
2. ocean; vast area of sea; great expanse of water
太平洋 - the Pacific Ocean
3. overseas; foreign; western; imported
洋人 - foreigner (especially a westerner or Caucasian)
洋蔥 - onion, lit=Western green onion
4. modern; stylish; fashionable
5. novel; unusual
6. (historical or Shanghainese Wu) foreign money; dollar
撥我三隻洋。 - tr=4peq+3ngu&1se tsaq&3hhian, Give me three dollars., ref=2007, Qian Nairong, 上海话大词典, page 29, SH
7. (Min, Hakka, often in place names) plain; flatland
8. (historical) (place, zh, river, in southern, p/Shaanxi, c/China)
葱
1. Simplified form of 蔥
2. usage. (Chinese-region variant characters, 蔥, 葱)