This is a topic that most pet owners are likely unaware of, and an event today reminded me of that.
My receptionist came back from lunch, and almost in passing asked me if onions are dangerous to cats. She had taken an onion off her hamburger and laid it on the counter since she doesn't like onions. Her cat jumped up and quickly ate it. She didn't think anything about it at the time, and almost didn't ask me. I had her immediately go back home, get her cat, and bring him in.
As soon as she arrived back at the hospital, I placed a feeding tube down his esophagus and gave him about 48cc of hydrogen peroxide. This is what we commonly give to induce vomiting, which he did within a few minutes. When a toxin is ingested, if you can get to the pet within about 60 minutes, you can often get them to throw it up and prevent absorption. You should never do this with caustic substances, but for many poisons and objects it works well.
Goober (yes, that's his name) threw up the food he had eaten, as well as several large pieces of onions. He was acting perfectly normal, though he was VERY unappreciative about having a rubber tube forced down his throat. We were able to get what appeared to be all of the onion out of his stomach before it passed, and I think he will be fine. If she hadn't brought him in quickly, we could have been dealing with a serious or fatal situation in a few days.
Why the big worry? Onions are very toxic to cats. Potentially fatally toxic, in fact. They cause abnormalities in the red blood cells leading to their destruction and severe anemia (called Heinz body anemia due to the structure created within the blood cells), and can also cause kidney failure. This can happen relatively quickly, and with only a small amount of onion. It has been known to happen after cats ate baby food with onion powder as an ingredient. Garlic is in the same plant family, and can cause the same toxicity. Cats have died from people giving them garlic tablets as a way to prevent fleas (which garlic does NOT do, and has been studied scientifically). These plants are also potentially toxic to some dogs, but the risk is nowhere nearly as serious as with cats.
So keep all of this in mind. NEVER give your cats anything with any onions or garlic in them. This is a very serious toxicity, and requires intense hospitalization and even blood transfusions once symptoms begin.