Onions have a characteristic of having layer after layer, even after you peel them. That's why, like in the film 'Shrek,' it's sometimes used to metaphorically represent a person's or a story's "complex inner self" or "hidden layers."

The sweet flavour in Korean food, especially Jajangmyeon, comes from fried onions. As such, it can be used to emphasise a key ingredient or to hint at a surprising twist in a story, like "it turns out there was a secret!" It can also be used as a compliment, meaning "the more I get to know you, the more new charms I discover."