Another update: I really hate Banfield and didn't want them fooling around with Capone with general anesthesia and pumping his stomach, etc. I managed to figure out who my regular vet's backup was and called them.
The vet was seemed to know a lot more about the toxicity of onions are was not terribly concerned. He said, based on Capone's size and the amount of onion he ate, that he didn't think it was enough to seriuosly hurt him. They told me to induce vomitting and watch him over night, but that he should be okay.
He vomitted again, still no onion, but he's acting completely like himself, so maybe he's one of the lucky ones that aren't affected too drastically.
Please keep my little guy in your thoughts. And please, please, remember to keep your pets away from chocolate, all onion (raw, cooked, dehydrated, etc.), grapes, and raisins.