I have read (and seen enough definitive, well-done evidence) that too many/much onions (the amount depending on dog's size and susceptibility) can give a dog hemolytic amemia, which is usually fatal. I have heard that too much in the way of garlic (which is in the same family as onion) can do the same, but have also heard that small amounts of garlic are good for dogs. I try to avoid onions totally, and mostly stay away from garlic, for my dog, who used to get garlic mixed in her food regularly, to no ill effect that I know of. I think, on this question, and until I see several well-designed and definitive studies published, I would feed very small amounts and not with regularity.

I too learned garlic is toxic to dogs if given too much or regulary.