Here.
It will blind you with tears
like a lover.
It will make your reflection
a wobbling photo of grief.
I am trying to be truthful.
Not a cute card or a kissogram.
I give you an onion.
Its fierce kiss will stay on your lips,
possessive and faithful
as we are,
for as long as we are.
Take it.
Its platinum loops shrink to a wedding-ring,
if you like.
Lethal.
The speaker is urging the lover to accept the gift. 
 One commentator saw this as a twist, the speaker’s shocking and commanding nature. However, the more likely interpretation is that the persona is plain-speaking and emphatic and rejects the flowery way, (‘please take this, darling’!), of offering the gift.  The imperative and contrasts with the line after next where Duffy use more tentative language, when she says “if you’d like”. 
 The interpretion of the tone of this depends on the reader. Its meaning would be clear if the poem were read aloud in performance.
Valentine