“If you hear an onion ring, answer it” (or “If an onion rings, answer it”) is a nonsense saying. “IF AN ONION RINGS ANSWER IT” was printed in the St. Louis (MO) Post-Dispatch o April 29, 1979. “If you hear an onion ring - answer it” was posted on the newsgroup comp.unix.questions on April 9, 1991.
   
“Why does Mr. Potato have a mobile phone?”/“Just in case Mr. Onion rings” is a related riddle about onion rings.
           
   
Newspapers.com
29 April 1979, St. Louis (MO) Post-Dispatch, “Our Own Oddities” by Ralph Graczak, comic sec., pg. ?:
IF AN ONION RINGS ANSWER IT
 
Newspapers.com
5 October 1982, The Daily Oklahoman (Oklahoma City, OK), “Off the Beat,” pg. 4N, col. 6”
MOORE—A message displayed on the drive-in restaurant reads: “If an onion rings, answer it.”
         
29 May 1989, Toronto (ON) Star, “A closed mouth will gather no feet” by George Gamester, pg. A2:
Okay, enough of the generalities. Let’s try to wind this up with a useful tip. Something we all should remember:
 
IF YOU SEE AN ONION RING - ANSWER IT!
 
Google Groups: comp.unix.questions
KSH-How do I make a variable run a function upon invocation?
J.B. Nicholson
Apr 9, 1991, 9:24:29 PM
(...)
| “If you hear an onion ring - answer it.” J.B. Nicholson |
 
Google Groups:  comp.sys.apple           
Apple II stuff
The Sneaky Devil
Jan 13, 1993, 4:24:26 PM
(...)
James A. Ertle |If you hear an onion ring, answer it.
     
Google Groups: rec.humor
one liners
Andy
Feb 24, 1997, 3:00:00 AM
(...)
If you hear an onion ring, pleas answer it.
 
PerlMonks
fortune
by BastardOperator (Monk)
on Nov 16, 2000 at 18:49 UTC
 
If you hear an onion ring, answer it.
   
Google Books
Gifted and Gone:
Proverbs, Quotations, Sayings and More
By Carol Giles
iUniverse, Inc.
2004
Pg. 64:
If you hear an onion ring, don’t answer.
(Unknown submitted by grammar school student)
 
Google Books
The power of promotional products:
How to motivate prospects, reward performance and create targeted promotions with residual value
By Maria Carlton and David Blaise
Hamilton, NZ: Phantom Pub.
2007
Pg. 74:
“If you hear an onion ring, answer it.”
   
Sioux City (IA) Journal
Slow-cooking dish calls on quote-worthy onions
Marcia Poole, Journal columnist
Posted: Tuesday, October 13, 2009 8:30 pm
(...)
Oh—and one more onion quote; this one’s from an anonymous source: “If you hear an onion ring, answer it.”
 
Facebook
KFC
September 26, 2019 ·
If you hear an onion ring, answer it 😉. Come down to any of our restaurants and savor our perfectly fried onion rings.
#PutARingOnIt
— with Sharon Ke.