Good article but wanted to add a few things.  First of all it is yellow onions that are high in quercetin.  It is the quercetin that gives these onions their yellow pigment.  This is why commercial quercetin is derived from yellow onion skins.

An old recipe for a cough syrup is simmering yellow onions with their skin to make a tea.  Then honey is added to the solution and it is simmered down in to a syrup.  Once cooled it can be used as a cough syrup as the quercetin is antihistamine and dries up sinus drainage.

They also left out that quercetin is a strong antiviral, which also helps to explain some of its anti-cancer properties.