Good advice from ahe other 2, but I want to add a bit. At 35 pounds that dog is probably fine with the onions, as long as it doesn't happen again. Their kidneys don't have an easy time with onions, but maybe mom-in-law used to feed her 15 pound Pomeranian onions, not knowing it was toxic to dogs. I assume dog felt sick later, but never enough for them to notice. So don't panic, but then again, different dogs react differently, so... Yes, vet. Just in case.
Call them. They don't read this
Would recommend bringing in for vomit induction to be on the safe side.
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