On onions, here is the reference:
ONIONS AND GARLIC
Both onions and garlic contain the toxic chemical thiosulphate. As thiosulphate levels build up in a dog’s body, they cause severe problems with the body’s red blood cells. Dogs can become ill after either eating one large amount of onion or regular smaller meals containing onions. All forms of onions are equally dangerous, whether they are raw, cooked, or included in a recipe, one reason to be especially careful with table scraps. Onions contain more of the toxic ingredient than garlic. While the potential for illness still exists, a dog would have to eat much larger quantities of garlic to become dangerously ill.
http://www.your-answer.com/dogs/pet-dogs-bad-foods.htm