Onions are toxic to dogs.
“These vegetables can cause red blood cell destruction (specifically, Heinz body formation) and result in anemia. Ingestions of onions or garlic of more than 0.5% of dog’s body weight are potentially toxic. For example, this would equate to a 30 lb dog ingesting about 2.5 ounces of onion or garlic.”
https://www.petpoisonhelpline.com/pet-safety-tips/are-onions-poisons-to-dogs/
Thank you for the information. Bertie has only eaten onions once and I was surprised he actually consumed them. He still says no to celery.