"Knows their onions" is a phrase I've not heard for some time; perhaps it's a little old fashioned these days.
Is it used much outside the UK?

I personally have never heard of it, but,

WELCOME

Recognize it, but never particularly associated it with any location in particular...

I hadn't heard this idiom before, but had a nice time exploring it. Thank you. A bit paradoxical as I wonder how to know thyself with a beginners mind. Ogres have layers. Does it have anything to do with

http://kundaliniandcelltowers.com/parable-of-the-onion.html