This poem expresses love and affection in the form of a conceit whereby the symbol of love being offered by the persona is an unconventional onion.
The poem is written in free verse using irregular stanzas to support its content and purpose, which is to reject traditional restrictive conventions such as marriage and other notions of love and to warn lovers that being overly possessive can have undesirable consequences.
While ostensibly a poem on the theme of love, Duffy deliberately avoids the use of language or imagery that we associate with this type of poetry.
Instead, the words are often stark and monosyllabic to allow her to present her ideas clearly and unambiguously.