A popular lawyer joke is:
 
Q: What’s the difference between a lawyer and an onion?
A: You cry when you cut up an onion.
 
The joke has been cited in print since at least 1991.
   
“What’s the difference between a politician and an onion?”/“No one sheds a tear when you chop a politician” is a similar riddle.
   
     
18 August 1991, The Sun (Baltimore, MD), “Surfeit of Lawyers” by Ray Jenkins, pg. 3E, col. 1:
What’s the difference between a lawyer and an onion? When you chop up an onion, you cry.
 
There must be a million lawyer jokes like that one floating around, and no doubt we can expect to hear them all now that Vice President Dan Quayle has declared war on lawyers.
   
Google Books 
Great Lawyer Stories:
from Courthouse to Jailhouse, Tall Tales, Jokes, and Anecdotes
By Bill Adler
New York, NY: Carol Publishing Group
1992  
Pg. 131:
What’s the difference between a lawyer and an onion?
When you chop an onion, you cry.
 
Google Groups: rec.sport.football.college
test
Scott Harper
9/21/92
(...)
Q. What’s the difference between an onion and a lawyer?
A. Nobody cries when you chop up a lawyer.
 
Google Books
Lawyers and Other Reptiles II :
The Appeal
By Jess M. Brallier
Chicago, IL: Contemporary Books
1996
Pg. 14:
Question: What’s the difference between a lawyer and an onion?
Answer: You cry when you cut up an onion.
 
Google Groups: uk.legal
What’s the difference between a lawyer and an onion?
Francesco Errante
1/4/02
What’s the difference between a lawyer and an onion?
You cry when you cut up an onion
 
Twitter
Simon F-Lessard‏
@simonfl
Replying to @pycon
@pycon What’s the difference between an onion and a lawyer?  You cry when you cut the onion.
10:11 AM - 15 Mar 2008
 
Twitter
Jefferson Kim‏
@jeffersonkim
Replying to @benlim
@benlim What’s the difference between an onion and a lawyer?  you cry when you cut an onion.
2:16 PM - 19 Aug 2008
 
Google Books
The World’s Funniest Lawyer Jokes:
A Caseload of Jurisprudential Jest
By Steven D. Price
New York, NY: Skyhorse Publishing
2011
Pg. 115:
Q: What’s the difference between a lawyer and an onion?
A: You cry when you cut up an onion.
 
Reddit—Jokes
Q: What’s the difference between an onion and a lawyer?
submitted June 4, 2017 by DoctorLobo60
A: You cry when you peel the skin off an onion.