I know my onions, as the well used saying goes. In French, there's the expression occupez-vous de vos oignons which means "mind your own business" in English but can be literally translated as "take care of your onions".
Know your onions however, means to be knowledgeable about a subject - a meaning which is clearly distinct from the French saying. Well in this case I know that this is what happens if you let your onions go to seed, just like me in my old age.
