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I am sorry to hear about this concern.
Well, yes, the onion content is less than ideal, but it wasn't ingested in large amounts. At this point, there is nothing you can do but get to your vet as soon as possible.
Symptoms of greatest concerns would be labored breathing, restlessness, lethargy, diarrhea and/or vomiting. If any of these symptoms are already present, I would go to an ER veterinarian now.
Otherwise, I would suspect your vet will immediately recommend a blood profile to evaluate for any hematologic changes that can occur with onion exposure. It is very important to recognize symptoms may not occur for 2 to 4 days after ingestion. So, even if doing okay, I would strongly recommend the vet visit as soon as possible.