Health expert Dr. Mercola lists onions amongst the most higly recommended vegetables.  He writes that:

“Onions are loaded with numerous health benefits, and scientists are still discovering how beneficial this vegetable really is. They’re a very good source of vitamin C and B6, iron, folate, and potassium. The manganese content in onions provides cold and flu relief with its anti-inflammatory abilities.

Two phytochemical (plant-derived nutrient) compounds in onions – allium and allyl disulphide – convert to allicin when the bulb is cut or crushed due to enzyme activation. Studies show these compounds to have cancer- and diabetes-fighting properties, while decreasing blood vessel stiffness by releasing nitric oxide. This can reduce blood pressure, inhibit platelet clot formation, and help decrease the risk of coronary artery disease, peripheral vascular diseases, and stroke.

Significant amounts of polyphenols (another phytochemical in onions) and an antioxidant flavonoid called quercetin (which has proven anti-cancer, anti-inflammatory, and anti-diabetic functions) account for the reputation onions have for disease prevention.

Luckily, cooking onions in soup doesn’t diminish their quercetin value – it simply transfers to the broth. The flavonoids in onions are more concentrated in the outer layers, so discard as little as possible.

Onions are one of Dr. Mercola’s most highly recommended vegetables.

James Kilkelly writes in his Irish garden blog that:

Onions are claimed to have anti-cancer properties and many believe that the evidence is stacked in this theories favour. They contain vitamin A and C, phosphorus and potassium.

As well as being beneficial for our health, alliums are also good companions for other plants in the garden. They are usually planted next to roses, carrots, beet and camomile to give them protection from disease and pests; if they are good enough for plants they can surely do a lot for us too! Therefore growing onions is immensely satisfying as they are so useful.
In cooking they are used as a condiment or seasoning; few main course dishes would taste quite as good without the addition of either onions or garlic.”