my dog ate an onion?

If you're concerned then contact your vet and/or poison control for reliable information, not strangers/children on YA. Anyone could say don't worry about it....but then should something happen to your dog as a result of this...you'd only have yourself to blame for failing to obtain the proper info from a reliable source.

Onions contain an ingredient called thiosulphate which is toxic to cats and dogs. The ingestion of onions causes a condition called hemolytic anemia, which is characterized by damage to the red blood cells. Onion toxicity can cause the red blood cells circulating through your pet's body to burst. copy/paste from pet med

Onions are toxic to dogs because of thiosulfates present in them. If he didn't ingest it, then he should be fine.

Are you CERTAIN about the amount?

actually, onions are very bad for dogs health. However, he is big so a little piece not have as much as an effect on him. Watch him when he goes to the bathroom and if his feces looks bloody or if he is having diarrhea I would suggest taking him to a vet as soon as possible.

My cat loves to get the produce off the counter and take bites, mushrooms, tomatos, potatoes, grapes, you name it, He has never been sick.

"Not surprisingly, pets actually have to eat the onions to get sick, but depending on their size, they may not have to eat much. One fourth of a cup can make a 20-pound dog sick while several cups may be needed to make a large dog sick. Cats are even more sensitive"
I think he will be fine. Doesn't sound like he ingested much. Just watch him tonight.

Not aware that onions are dangerous to dogs. Odds are if he would have spit it out long before he would have gotten enough in to him to cause any harm due to the taste.