The onion represents the harsh reality of purity, which must in its faculty, eliminate persistent contaminants. As opposed to an apple, or a loaf of bread, an onion will not spoil for an unusually long period of time. Hence, we see how the onion is far stronger than most forms of bacteria.
Moreover, we witness in the course of this book, how impurities have become archetypes of evil and more accurately, immoral behavior. As such, the onion is pure, healthy and virginal (untouched). It is the embodiment of dedicated virtue and rectitude. It is not fun, but dam decent, nevertheless. The onion reminds us about the difficulty required to stay the moral path. In fact, one may make the biblical analogy, that once Adam and Eve ate the apple, mankind has been forced to eat onions ever since!
This alliaceous plant enjoys so high a reputation that a religious sect was devoted to its cultivation. Its bulbous shape, strong smell and layers of skin one on top of the other provide subjects for symbolic interpretation. Ramakrishna compares the laminated structure and lack of a central core with the structure of the ego. Spiritual experiences strip it away layer by layer until there is voidness and then there is no barrier to the Universal Spirit and union with Brahma.
On a magical level, the Ancient Egyptians protected themselves against certain diseases with onion stalks, while Plutarch says that the Romans forbade the plant because they believed that it waxed while the Moon waned and its smell weakened the life force. Aphrodisiac properties have also been attributed to it, as much because of what it suggests to the imagination as because of its chemical composition.