Onions are toxic to dogs so the dogs should not be fed or eat any onions.
All parts of the onions are toxic to the dogs health and can be deadly if they eat enough of the onions.
If your dog happened to eat just a small bite of onion then likely nothing will happen to the dog.
However when the dog eats a whole onion or more than one onion then the dog could possibly die or suffer at least serious health issues that would require a vet visit.
When a dog eats onions then the ingestion of onions can cause conditions called hemolytic anemia, Heinz body anemia, and methemoglobinemia which are all manifestation of damage to red blood cells.
If your dog eats more than a small bite of onion then you should keep an eye on the dog and if they experience problems call the vet or take the dog to the vet.