Know one's onions

Somewhat surprisingly, the origin of this expression, which means to know one’s stuff, is American from the 1920s. At the time, there was a whole host of expressions that meant the same thing - know your oats, know your apples, know your eggs etc where virtually anything could be replaced by onions. Some of these, like the onions expression, have survived but no one can tell us why. Perhaps just too many everyday things can be substituted for stuff. Another theory but not proven, is that it derives from rhyming slang, onion rings/things but Americans as a general rule are not into rhyming slang so this origin is most unlikely.