”It promises light like the careful undressing of love.” - Extended Metaphor, Simile - “light” captures the hopefulness and positivity surrounding a new relationship. “undressing” suggests the deepening of an emotional bond as partners strip away the layers of their personality.
“It will blind you with tears like a lover.” - Extended Metaphor, Simile - Highlights the negatives of a relationship. Suggests relationships may cause pain and upset, just as the chopping of an onion can bring tears to your eyes.
”Its fierce kiss will stay on your lips,
possessive and faithful” - Extended Metaphor, Juxtaposition - Just as the taste of onion can linger, so too can the memory of a passionate kiss. ‘possessive’ implies a controlling relationship.