Quercetin is an anti-inflammatory and anti-oxidant flavonoid molecule found in many fruits, vegetables, and leaves. Foods high in quercetin include berries, apples, onions, kale, and broccoli. It can help reduce oxidative stress and therefore potentially reduce the risk of cancer, as well as prevent inflammation by way of inhibiting mast cell degranulation.
In this video, I extract some quercetin from onion skins. I do not recommend that anyone attempting this process try to ingest or otherwise use any products they extract for medicinal reasons or as a supplement.