Onion inhibits the blood platelets from clogging together and forming a thrombus, which may be harmful to the human body. Steam cooking, on the other hand, reverses this action of the onion.

Raw onions are blessed with the ability to prevent platelet aggregation (clumping together of blood platelets), but when steamed they lose their ability to do so, found Emilie Hansen at the Department of Health Professions, Metropolitan State University of Denver, Denver, USA and her colleagues, in this study.
The main objective of this study was to examine how cooking affected the antiplatelet activity of onion in multiple human blood donors. They also studied the polyphenol concentration and change in soluble solids after cooking of onions.
Onions were quartered and steamed for 0, 1, 3, 6, 10 and 15 minutes respectively and using in vitro blood aggregometry, their efficiency at exhibiting anti-platelet activity was tested. The results showed that the onions steamed for 3 and 6 minutes demonstrated low or no anti-platelet activity. However, anti-platelet activities of onions were reversed at after 10 minutes of steaming, and the onions actually stimulated platelet activity and encouraged possible thrombus formation.
It was thus, concluded that onions managed to retain their anti-platelet activity only till a few minutes of cooking. Patients suffering from thrombosis should consider consuming raw onions than cooked ones.
Total polyphenolic concentration and total dissolved solids content were, however, not affected by steaming time.
Reference: Steam-cooking rapidly destroys and reverses onion-induced antiplatelet activity; Emilie Hansen et al; BMC Nutrition Journal 2012