"How it turns out" is also often phrased in the form of, "tell me how it went". "turn" and "went" are directly related, as "went" comes from an old word "wend", which means "turn".
Isn't that interesting? When you ask how something went, you are literally asking how something "turned" out.
Went is the past tense of go. Turn represents just that, rotation or revolution, a thing going.
This is an American phrase, first recorded in the May 1922 edition of Harpers Magazine:
"Mr. Roberts knows his onions, all right."
According to World Wide Words, this had nothing to do with any Mr. Onions, but:
The crucial fact is that the expression isn’t British
but American, first recorded in the magazine
Harper’s Bazaar in March 1922. It was one of a set
of such phrases, all with the sense of knowing
one’s stuff, or being highly knowledgeable in a
particular field, that circulated in the 1920s. Others were to know one’s oats, to know one’s
oil, to know one’s apples, to know one’s eggs,
and even to know one’s sweet potatoes (which
appeared in a cartoon by T A Dorgan in 1928).
You may notice certain similarities between the
substances mentioned, most being foods and most having names that start with a vowel.
They contain much of the verbal inventiveness
and mildly juvenile wordplay that characterises
another American linguistic fad of the flapper
period, that of describing something excellent of
its kind in terms of an area of an animal’s
anatomy (elephant’s instep, gnat’s elbows and about a hundred others — see my piece on bee’s knees for more).
As with bee’s knees, one of these multifarious
forms eventually triumphed and became a
catchphrase that has survived to the present day.
The Phrase Finder agrees:
Other phrases that refer to
knowing - 'know the ropes', 'doesn't know shit from Shinola' etc. allude to specific items as
the focus of the knowledge.
Other 1920s variants of 'know
your onions' are 'know your oil/
oats/apples' etc. The only one that caught on and
is still in common use is 'know your onions'. So, why onions? Well, as the citation above asks -
why not? Explanations that relate the phrase to
knowledgeable vegetable gardeners, or even to C.
T. or S. G. Onions, are just trying too hard. 1920s
America was a breeding ground for wacky
phrases (see the bee's knees for some examples) and this is just another of those.
Edit: A tantalising snippet in Google Books shows this may have been used in 1908 in a humorous poem in The Postal Record (Volumes 21-22 - Page 27). It' shown in the summary, and is interesting as the year 1908 is also shown. Care must be taken with snippets, as they're sometimes incorrectly dated, but here it is anyway:
But, never mind; Billy knows his onions, He Is not troubled with corns or bunions. He travels along at a good, fair gait; Unless the roads are bad, he Is never late. O. 8. WHITE. WHkesbarre, January 1, 1908. West Hoboken, N. J. At the regular meeting of Branch 1065, held on January 10, 1908, it was honored by the presence of Brother Kelly, President of our National Association.
Best Answer
The OED has this phrase meaning to "(to go) very quickly" from 1921:
However, I found some antedatings.
First, from August 17, 1895 in the Evening Star (Washington DC, Page 15, Image 15), in an article titled "COWBOYS AT WORK / Hamlin Harland Gives His Impression of a Round-Up. / THE CRUELTY OF BRANDING / Some Stirring Encounters Between Man and Beast. / WITH THE COW BOSS":
Placing the quotation on the map, the report itself is from "SALEDA, August 4, 1895" and begins "At Cripple Creek mining camp...". There's both a Salida (note spelling) and Cripple Creek in the state of Colorado, just 50 miles from each other as the bat flies.
Next, two antedatings via the American Dialect Society mailing list. From Stephen Goranson:
From Fred Shapiro:
Finally, Dialect Notes (Volume III, Part V, 1909) is good as it gives a descriptive reason for the phrase: