know one's onions

Verb

know one's onions (third-person singular simple present knows one's onions, present participle knowing one's onions, simple past knew one's onions, past participle known one's onions)

1. (informal) Synonym of know one's stuff: to be knowledgeable about a given subject or task.
   • Joe really knows his onions when it comes to cooking.
   • Sam Tyler: Well, our first priority is to make sure the thieves don't strike again. It'll be tricky, 'cause these sorts of gangs rarely have just one target on the go. They've usually researched half a dozen possibilities, waiting for the most opportune circumstances on any of them. / Harry Woolf...2007 February 20, Chris Chibnall, Life on Mars, season 2, episode 2

   Synonyms:

   know one's stuff