If we tease that apart a bit, kabbalistically, we observe first and foremost that an onion has layers; that it is a food; and that the word onion sounds like "a nun". From this we can derive that the piece of commentary referenced invokes a sense of layered meaning, is nourishing, and inspires religious devotion.

Digging, pun intended, a little deeper, we find that the etymological root (again intended) of the word is the Latin unio, unionis. Obviously "union" is right there –and this is not a coincidence because nothing is ever a coincidence, in Kabbalah– so we must understand that these layers of meaning are unified. Furthermore, the Latin word also is used to demote a large pearl, harkening to the "pearl of great price" allegorically mentioned by Christ as a stand in for spiritual wisdom; seeming to imply that not only is the above all unified but spiritually sound from the perspective of Christian soteriology.