It could also indicate that she feels that the image of the onion as 'a moon wrapped in dark paper' shows that a relationship unfolds, different aspects of a person are revealed, leading back to the idea of what you may see initially is not what you will get ultimately.
'It will blind you with tears'. Onions are well known for making people cry - is she illustrating her discontent with the relationship or possibly reflecting how the relationship makes her feel, i.e. like crying? It seems the onion as a gift is symbolising an unhappy relationship.
'Its fierce kiss will stay on your lips'. Raw onions leave a strong taste in one's mouth and the heat of a raw onion is strong and a shock to the senses. This could be seen as a passionate statement - when people are described as a loving couple - they may share 'fierce' kisses, however, in this case the image is linked with an onion which seems almost poisonous - something you would not want to taste, like an unhappy relationship, something that you do not want to linger.
'Its platinum loops shrink to a wedding ring'. A metaphor is used to link this relationship to an image of a loving relationship i.e. marriage, however the use of the image of the onion shrinking suggests that marriage would be a consequence of shrinking emotions, an anti-climax, an unhappy ending. This is not a usual image of marriage or a content and loving relationship.
'Its scent will cling to your fingers, cling to your knife'. When you chop up an onion the aroma is very pungent and clings to your skin and the chopping knife and board. When a relationship is in trouble people describe themselves as feeling 'cut up' as though they had been cut with a knife. Ann Duffy uses this metaphor to describe the end of a loving relationship.
The War Photographer is a similar poem by Ann Duffy, it tells the story of a War Photographer and his repetitious, and shocking life.
'Spools of suffering'. Spools refer to the photographs he has taken, illustrating the suffering caused by war and there is a reference to the rows of gravestones in cemeteries.
'All flesh is grass' is referring to a man's mortality, we came from the earth and when we die we return to the earth.
'Running children in a nightmare heat'. Upon first glance this suggests children running around on a hot day, however in the context of war it describes children running in panic and terror in the heat of a day that has turned into a nightmare. The 'nightmare heat' could also be referring to the Napalm bombs that were used by the US in the Vietnam War.
'A hundred of agonies in black and white' describes the suffering of people in countries at war captured on film by the war photographer.