What's the meaning of "knowing one's onions"?

[Xavierka]

English Language Tests, Intermediate level

ESL/EFL Test #384 "English Slang Idioms (115)", question 2

"Jake is a good employee. That guy sure knows his .......... There isn't anything about how this place is ran that he doesn't know about," Marcie told Melissa.

(a) onions
(b) ideas
(c) feelings
(d) shoes

English Language Tests, Intermediate level

ESL/EFL Test #384 "English Slang Idioms (115)", answer 2

"Jake is a good employee. That guy sure knows his onions. There isn't anything about how this place is ran that he doesn't know about," Marcie told Melissa.

Correct answer: (a) onions

Your answer was: correct
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What's the meaning of "knowing one's onions"?

[Mister Micawber]

.
Be knowledgeable.
.
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Native English teacher at Mister Micawber's

[Zishuli]

Hi,

I believe there is a grammatical error in this text-"about how this place is ran ".
Shouldn't we say " this place is run' ?
If it is actually a mistake, there is another one in question no.3 " things are ran".

[Esl_Expert]

Hello Zishuli,

You are right. The past participle of the irregular verb "run" is "run". The use of "ran" as a past participle would be marked as wrong in any reputable grammar test.

In the passive voice, the verb "run" should use a form of the verb "be" and the past participle ("run"). For example:
- I know only too well how this place is run.
- Three different simulations were run.

You would also use the past participle ("run") to build the perfect tenses:
- We have already run three different simulations.

The word "ran" is used for the simple past tense, active voice:
- He ran the place with an iron hand.
- We ran three different simulations.
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"Nearly all men can stand adversity, but if you want to test a man's character, give him power."
— Abraham Lincoln

[Torsten]