I'm Not Gonna Write You A Love Poem

Valentine by Carol Ann Duffy

Not a red rose or a satin heart.

I give you an onion.
It is a moon wrapped in brown paper.
It promises light
like the careful undressing of love.

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.
Its scent will cling to your fingers,
cling to your knife.

http://famouspoetsandpoems.com/poets/carol_ann_duffy/poems/8116

She is saying that love can be unpleasant like an onion but it will always permeate everything like an onion's smell and sometimes sting like a onion too.

"It is a moon wrapped in brown paper" means an onion, like love, will slowly get rid of its plainess and show its brilliance.