Not a red rose or satin heart - typical valentine gifts, which is "not" what this poem is about. Draws attention to poem's main message - challengingideas about love
I give you an onion - Extended metaphor - compares love/ relationships to onion. Duffy chooses something unusual to challenge ideas of conventional love
i give you an onion
not a red rose or satin heart
moon wrapped in brownpaper - inside of onion is white just like moon, representing romance and beauty. covered with brown shows skin of onion as wrapping paper hiding beauty. stereotypical romantic image is hidden