An important pharmacological aspect of onion is the ability to inhibit platelet aggregation.
However, this property is destroyed by steaming....in fact cooking might actually render the onion unhealthy!
The study is available in full at
ConclusionsIn the present study, antiplatelet activity induced by onion extract was destroyed between 3 and 6 min of steaming. At 10 min of steaming, cooked onions stimulated platelet activity, and cooked extracts had the potential to reverse the inhibitory effect on blood platelets by 25%.Thus, possible negative health implications of eating onions in the cooked form were revealed. Because OIAA was lost and reversed with steam-cooking, consumers may want to minimize onion cooking time, choose onions with high pungency for cooking practices, and/or consume raw onions on a regular basis in order to preserve OIAA.