[Ladymom]

Yes, onions are toxic:

Onions/Garlic:

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. Symptoms include Hemolytic Anemia, labored breathing, liver damage, vomiting, diarrhea, discolored urine.

The poisoning occurs a few days after the pet has eaten the onion. All forms of onion can be a problem including dehydrated onions, raw onions, cooked onions and table scraps containing cooked onions and/or garlic. Left over pizza, Chinese dishes and commercial baby food containing onion, sometimes fed as a supplement to young pets, can cause illness.

While garlic also contains the toxic ingredient thiosulphate, it seems that garlic is less toxic and large amounts would need to be eaten to cause illness.

Toxic Foods and Plants for Dogs