If interested in the origins of the onion metaphor, I believe credit goes to Henrik Ibsen where he first successfully utilized it in his classic play,
Peer Gynt:
The play focuses on the problems of choice, and of identity. "What is it to be one self", Peer asks in the end, and gets the answer: "to overcome one´s self". In a central scene, we find Peer pondering his identity, and picks an onion to look for the core of it. He declares himself an onion, and in the process finds nothing but layers. Who has he really been? Philosophically, the existential core of the play should be plain to see. Related to this theme, we also find the old riddle of the Sphinx. Much of the later dialogue revolves around riddles, and Man's purpose. The play is considered based on the medieval morality play, and Peer is related to the main Character Everyman, who has to atone and make up for his life when unexpectedly facing death.
The themes from the play surfaced again in the book The Neverending Story by Michael Ende. Many topics and even scenes from the play are referred to in the book. Peer Gynt's life with the trolls and as a troll also provides some interesting metaphors for life as a drunk: "Out there, where sky shines, humans say: 'To thyself be true.' In here, trolls say: 'Be true to yourself-ish.'"