To know your onions

Li: Hello, I'm Li and welcome to The English We Speak, and today I'm here with Rob. Hi Rob.
Rob: Hello Li. Now come on Li, concentrate, I need you to help me finish this quiz in the newspaper. If I get all the questions right, I might win a holiday.
Li: Oh right. I love quizzes, especially if it means you can win a holiday! OK Rob, what's your next question?
Rob: Right. It says here, what is the capital city of Chile?
Li: Easy. Santiago.
Rob: Good. Next, what is the name of the world's largest ocean?
Li: The Pacific—of course!
Rob: Wow—you're good Li. You really know your onions.
Li: I know my onions? Err, was there a question about onions?!
Rob: No Li!
Li: Good, because that is one thing I don't know anything about, onions—except that they make me cry.
Rob: Don't cry Li. It's a compliment. If someone says you know your onions, it means you are experienced in something or you know a lot about a particular subject. So really, it means you're clever!
Li: Oh really?! But why 'onions'?
Rob: It is a strange term and some people say it comes from a man called S.G. Onions who made fake coins to help teach children about money. If they learnt about money they would know their onions.
Li: I see. But I suppose now we just use it as a silly expression.
Rob: We do. Like this ... 
Li: So knowing your onions means knowing a lot about something. Well that describes me very well then!
Rob: Hmm. Now another way of saying it, is that you really know your stuff.
Li: Stuff. You mean like this ... 
Rob: Well, knowing your onions or knowing your stuff, is very useful when you're doing a quiz like this. So Li, here's another question Li. Complete the name of this flavour of crisp ... 'Cheese and something ... ?'
Li: Oh, that's tricky ... cheese and ham? Cheese and apple? I've got it! It's cheese and onion crisps!
Rob: Brilliant Li. You really do know your onions. And now I might win a holiday.
Li: That's great but who are you going to take with you?
Rob: Probably my girlfriend.
Li: Oh right. I didn't want to go with you anyway—you have got onion breath!
Rob: Oh dear. Join us again soon for another programme about everyday English sayings.
Li: Bye!