Onions are low in calories and in most vitamins and minerals (although they do supply a little calcium, iron, and potassium). However, the many varieties of this plant family including scallions, leeks, shallots and garlic as well as onions themselves, are rich sources of a number of phytonutrients. They contain allyl sulfides (sulfur compounds that may lower blood pressure and discourage tumor growth), quercetin (a flavonoid with high antioxidant activity), and saponins (substances connected with cholesterol-lowering and tumor inhibition).
Onions originated in prehistoric times and were widely consumed in ancient Egypt, Greece, and Rome. By the 17th century, Europeans were enjoying them as a salad ingredient and as a breakfast "health" food. Today, onions rank sixth among the world's leading vegetable crops.
As a rule, the large, mild spring and summer onions are good for eating raw or for cooked dishes in which you want a subtle flavour. The crisp, assertive character of storage onions makes them better suited for dishes that require long cooking, since they can hold their flavour. An onion's flavour is determined by its variety, and also by the soil and climatic conditions where it grows. Consequently, onions with the same appearance can taste considerably different, depending on where and when they were grown. So you may have to experiment, particularly when it comes to choosing the mildest onions.
Whatever type you choose, look for ones that feel dry and solid all over, with no soft spots (a sign of rot) or sprouts. The skin around the neck should be tightly closed, and the outer skin should have a crackly feel and a shiny appearance. Whole onions should smell mild, even those that are pungent when you cut into them; a strong odour is a sign of decay. Also avoid onions with green areas, which can taste unpleasant, or with dark patches, which may indicate mould.
Whole onions should be kept in a cool, dry, open space, away from bright light (which can turn their flavour bitter.) They do best in an area that allows plenty of air to circulate around them, so either spread them out in a single layer or hang them in a basket. Onions will absorb moisture, causing them to spoil more quickly, so don't store them under a sink (which can be damp) or place them near potatoes, which give off moisture and produce a gas that causes onions to spoil more quickly.
Chopping or slicing an onion brings its sulphur containing amino acids into contact with enzymes to form volatile compounds, one of which strikes the tongue, while another irritates the eye, apparently by turning into sulphuric acid. The older an allium is, the stronger these compounds become. Fortunately for our taste buds, cooking produces further chemical changes that render them much milder. (Some of the odour compounds appear to be converted into a substance that is 50 to 70 times sweeter than table sugar.)
Onions can be sliced, chopped, diced, or grated, but first they must be peeled. To make this task easier (if you need to prepare a large quantity of onions), trim off the tops and bottoms and place the onions in boiling water for about a minute. Drain them and pull off the outer skin, which should be loose, then peel off the slippery membrane underneath.
Although some recipes call for raw onions to be cooked with other ingredients, others require them to be cooked beforehand. Virtually every cooking method has been used with onions; since they are low in nutrients, the length of cooking time is not a problem.