This poem is not about the meaning of love so much as it is a description of what the author feels love is. It isn't anything superficial, like the gifts we usually give on Valentine's day ("a red rose or a satin heart"). Instead, it is...an onion. She gives her love an onion as a symbol of love as her valentine. Why?
"It is a moon wrapped in brown paper." Real, abiding love isn't fancy; it is cloaked simply. It's beautiful inside, but it isn't easy to recognize, at first glance, as anything special.
"It promises light / like the careful undressing of love." This undressing can be both literal and allegorical. Literally, undressing one's lover promises joy (light); allegorically, getting to know one's lover brings discovery and joy.
An onion will also "blind you with tears / like a lover." Again, unlike a box of chocolates, love isn't a shallow thing, all happiness and pleasure. It brings grief sometimes, too.
Perhaps Valentine's day is meant for the "kittens and rainbows" approach to love, but Duffy says, "I am trying to be truthful." Love is "Not a cute card or a kissogram."
An onion is more like love than any of these superficial symbols, she argues. "Its fierce kiss will stay on your lips, / possessive and faithful." Like the bite of an onion (a food that bites you back), love's taste and sting stays with you.
She continues to offer her lover the onion, urging him to take it. It is composed of concentric rings, "platinum loops" that will "shrink to a wedding-ring,.... if you like." This is a masked proposal, but she invites him to love her with his eyes open, with a full understanding of what love entails. It is, she says, "Lethal." Again, she isn't a sucker for the romantic ideals, but for reality. Love can be frightening, poison to the wrong person. It's unshakable; "Its scent will cling to your fingers, / cling to your knife." It will become a part of who you are, for better or for worse.
The only romantic images she uses are "moon" and "kiss"; the rest are blunt and stark. She doesn't mean to scare him away, but she doesn't want superficial love.