Onions cause Heinz body anemia in dogs and cats. Cats are much more sensitive, but enough onions can also cause serious issues or death in dogs.
http://mmpinschersminiature.homestea...NG_Onions2.htm
Other things I don't give dogs are: cooked bones, corn cobs, cooked fat (such as fat trimmings off of your plate, chicken skin, etc), chocolate, alcohol. I keep the cruficerous veggies to a minimum as well, since they can suppress thyroid function in dogs. (cabbage, broccoli, cauliflower, bok choy to name a few)