According to Stephen J Ettinger, D.V.M and Edward C. Fieldman, D.V.M.'s book:
"Textbook of Veterinary Internal Medicine", vol. 2 pg 1884. "The most common cause of Heinz Body hemolysis in dogs is related to ingestion of onions.
Their toxic effect appears to be equal whether raw, cooked, or dehydrated.
Although the toxicity has been known for over 50 years, animal owners still unknowingly feed onions to dogs, usually as part of table scraps. The hemolytic episode may be difficult to correlate with onion ingestion because it occurs several days later."