Sulphur containing compounds in onions are capable of breaking down mucous and can be used as natural decongestants/ expectorants. An effective traditional remedy during upper respiratory tract infections is to peel and cut up an onion and leave it on a saucer by your bedside overnight.
Sulphur containing amino acids such as cysteine in garlic and onions may promote the bioavailability of iron and zinc from food grains (eg rice, sorghum, chickpea and green gram pulses).
Smita Gautam, Kalpana Platel, Krishnapura Srinivasan (2010). Higher Bioaccessibility of Iron and Zinc from Food Grains in the Presence of Garlic and Onion. J. Agric. Food Chem. 58 (14):8426–8429