He knows his onions

I think I'm losing it. Random phrases and locations pop into my head at the oddest of times, some of which are from several decades ago. 
Which is perhaps why "He knows his onions" popped in there. 
A little research and.....
phrases.org.UK tells us that Know your onions is in fact an American phrase. There are many references to it in print there from the 1920s onward, but none in the UK or elsewhere until the middle of the century; for example, this from Harper's Magazine, March 1922:
"Mr. Roberts knows his onions, all right."

But I'm not Mr Roberts

Ah well..