Onions are perhaps the universal vegetable; they occur in almost every cuisine across the globe, and it comes as no surprise that they were one of the first vegetables to be planted: The Egyptians grew (and worshiped) them, the Israelites mourned their absence during the Exodus (Numbers 11:5), and the Romans, who consumed them avidly, made certain that they were readily available throughout the Empire.
The Romans also considered onions to be curative (nothing new; the Egyptians considered them to be a symbol of Eternal life), and Pliny the Elder notes that they were used to treat a variety of conditions, including problems with vision, sleeplessness, mouth sores, dog bites, dysentery, and back aches. Some of these conditions now are cured by other means, but the Romans were right in thinking that onions promote health: they're mildly antiseptic, and are also one of the best sources of quercetin, a flavenoid (a type of antioxidant) that protects against a variety of ills, including cataracts, cardiovascular disease, and cancer; in addition, their organosulfer compounds help lower blood pressure and cholesterol, while the more pungent varieties also inhibit the platelet aggregation that is behind the formation of thromboses.
In addition to all this they are low in calories (about 30 per 1/2 cup serving), and are a good source of vitamins C and B6. And they contain no fat.
So they have a lot going for them. On the down side there is that frustrating tendency of raw onions to rise up from the stomach and produce onion breath; this problem can be alleviated by selecting sweeter onions (Tropea if you're in Italy, or a variety of onions including Vidalias elsewhere), or, if one is already suffering from onion breath, by chewing a slice of lemon peel or a sprig of parsley. The problem rarely arises if the onions were cooked. The second problem with onions is that they can be windy; I haven't found any remedy, though the milder varieties seem to be less potent in this regard. A final important warning is that onions are toxic to both dogs and cats, causing a condition in called Heinz body anemia, which can become quite serious. So don't give your onions to your pets.
Purchasing, Storing, and Use | Oniony Recipes