09 Jun 2017 --- In the first study to examine how effective Ontario-grown onions are at killing cancer cells, University of Guelph researchers have found that not all onions are created equal and that red onions have the strongest cancer-fighting power. Onions as a superfood are still not well known. But they contain one of the highest concentrations of quercetin, a type of flavonoid, and Ontario onions boast particularly high levels of the compound compared to some parts of the world.