We just looked into why onions are toxic to dogs (we heard it on a TV show).
Onions are very bad, garlic is bad, but not as toxic as onions.
"Onion and garlic poisoning:
Onions and garlic are other dangerous food ingredients that cause sickness in dogs, cats and also livestock. Onions and garlic contain the toxic ingredient thiosulphate. Onions are more of a danger.
Pets affected by onion toxicity will develop haemolytic anaemia, where the pet’s red blood cells burst while circulating in its body. "