Foods rich in quercetin. Quercetin is a fatty acid with very effective antioxidant properties, and is believed to be a powerful anticancer agent. Up to 10% of a red onion's dry weight is made up of quercetin, with the next highest source being red grapes, then broccoli. White onions contain little or no quercetin. This substance also protects arteries, discourages blood clots, kills bacteria, inhibits viruses, and is an anti-inflammatory and anti-allergenic agent. These fruit and vegetables are also rich in vitamins and minerals.