Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Onions in a dream indicate the appearance of hidden matters and the revealing of secrets. For a healthy dreamer, it is sickness and fatigue, and for the sick, recovery. Eating it indicates evil, worry, and unlawful money.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Onions indicate unlawful money, gossip, and ugly speech. Eating it cooked indicates repentance and returning to God. A few onions indicate seeking good that is followed by evil.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
Onions are interpreted as worry, grief, and crying. Eating them indicates hearing disliked words from relatives. Peeling them indicates flattering a lowly man.