While it is tasty and harmless to humans, onions are very toxic to cats. The following is a list of foods that cats should never eat:
Onions, Garlic, & Related Root Vegetables (gravy and soups are made with these - don't feed from your plate to your pet!)
Onions and Garlic destroys red blood cells in the cat, causing an anemia.
Green Tomatoes, Green (raw Potatoes)
Can cause violent lower gastrointestinal symptoms. The leaves and stems are particularly toxic. (Tomatoes in pet foods are ripe, and should cause no concern because they appear in relatively small amounts)
It's becoming more widely known that chocolate is very toxic to both cats and dogs: chocolate toxicity
Grapes and Raisins
These foods' toxicity has mainly been found in dogs, in quantities of varying amounts.
Although milk is not toxic to cats, it may have adverse effects such as lactose intolerance. Cats do not need milk. If they love it, cream may be fine, given less often (The more fat in the milk, the less lactose.) Another compromise is CatSip, a product made from skim milk with an enzyme added that helps the digestion of lactose.