Onions are healthy!
Research shows that onions are healthy, they contain a high amount of quercetin which helps you body to protect itself against hart and blood vessel diseases. Quercetin is a type of antioxident which is also found in the red wine and apples.
Besides antioxidants onions also contain a lot of antioxidants, which make free radicals ineffective. Free radicals are involved in the development of several types of cancer and other diseases. Therefore it is important to ensure that your body has enough antioxidants available. Vitamine C, vitame E and beta carotene are examples. Onions contain five to ten times as much antioxidants than other vegetables.
Furthermore onions contain, like all other vegetables, very little calories and no fat. With an onion in the salad, or sauted onions with meat you add a lot of flavour but hardly any calories. 100 grams of onion contains as much fibers as a slice of bread.
More facts about onions are on: http://programma.groentenenfruit.nl/ui (also source).