I remember reading a top-10 ways to fight cancer list, and one of them was to eat plenty of alliums. No other food was mentioned. So I did some digging and while lots of vegetables have some protective effect, alliums were more strongly coorelated with anti-cancer than any other food by a country mile.

I haven't looked recently, I just remembered that. Maybe sauteed onions develop sugars. But I wouldn't want to remove them from my diet.

Actually I just poked around a bit and garlic is best, then leeks, then onions. Cruciferous/Brassica veg comes next. Most other vegetables don't have a strong effect against cancer. In vitro, garlic completely stops the growth of breast, brain, lung, pancreas, prostate and stomach cancer cells. In epidemiological studies, "alliums" as a group came out far ahead of the rest and I wish I could find that graph but I can't find it right now.