Some people find them both ‘stinky’, some-other ‘delicious’ but the fact of the matter is that both are essential.
No one can live in complete falsehood and it is extremely hard to put up a decent meal without using any onions.

There is more.
Truth, exactly like onions, comes in layers.
You peel one, think your knowledge has deepened, peel another one, then another one… and end up having nothing….
The onion is gone and if you dig deep enough into any problem you reach a level which seems incomprehensible for a rational mind – the only visible explanation being pure hazard, something we are not yet prepared to accept.
Not even one of the most gifted scientific minds: ““All matter originates and exists only by virtue of a force which brings the particle of an atom to vibration and holds this most minute solar system of the atom together.
We must assume behind this force the existence of a conscious and intelligent mind. This mind is the matrix of all matter.”  Max Planck” After he discovered that the world is made up of small bits and pieces (quanta) coming together, at first, in a purely haphazardous way, he still needs to find out what lies in between those bits and pieces. So strong that need that he had to come up with a ‘spiritual’ explanation if he couldn’t find a rational/scientific one that didn’t involve pure chance.

And the third thing that truth and onions have in common is man.
Or, more exactly, a self-conscious observer/operator to search for the truth while gorging on peeled onions.