Onion has been reported as one of the major sources of dietary flavonoids in many countries, contributing to a large extent to the overall intake of flavonoids.

Two flavonoid classes are mainly found in onion, the anthocyanins and flavonols:

Anthocyanins impart a red/purple color to some onion varieties. Scientific studies, such as cell culture studies, animal models, and human clinical trials, show that anthocyanins possess antioxidative and antimicrobial activities, improve visual and neurological health, and protect against various non-communicable diseases.

Flavonols such as quercetin are responsible for the yellow and brown skins of many other onion varieties. Quercetin 4′-glucoside and quercetin 3,4′-diglucoside are reported as the main flavonols in onions, accounting for about 80–95% of total flavonols. Quercetin has antioxidant and anti-inflammatory effects which might help reduce inflammation, kill cancer cells, control blood sugar, and help prevent heart disease.
Quercetin may have benefit for some airway infections, but there is no good evidence to support using it for COVID-19.

(Sources of Information: ncbi.nlm.nih.gov, onions-usa.org & intechopen.com)