I have always thought that onions and garlic were health foods...
But apparently, they can be toxic and are known to cause anemia in animals:
Onions and other members of the Allium family may cause bad breath, indigestion and acid reflux (15), anemia, reduced blood clotting, or even death. Pregnant women should avoid the Alliuin family because its members have been known to cause accidental abortions in humans (13). Nursing women should not eat them because the chemical Allicin enters into their breast milk and disturbs a baby's ability to breast feed (13).
Those at risk for major bleeding, such as those soon to undergo surgery or enter into labor are urged not to eat members of the Allium family because they have been shown to prevent the clotting of open wounds (14).
Disclaimer: the site seems to promote some vegan raw food diet which I obviously don't support or recommend, but it's true that onions are dangerous for pets, just google about onion-caused anemia in dogs. And he lists his sources, too.
P.S. OFF TOPIC
I just checked my comments and found some in moderation including one by Nuke1776 and posted them. Any comment on a post older than 10 days automatically goes into moderation and unfortunately, I often forget to check them. Excuses!