No – Onions are toxic to rabbits!
Why should I not feed onions to my rabbit?
Onions contain disulfides (also the cause of their strong smell), which can rupture their red blood cells resulting in haemolytic anemia. Other effects from ingesting onions may include diarrhea, ataxia. Onions can also suppress your bunnies immune responses, which can be fatal if they have any allergies.
No – rabbits should never eat red onions.