The Basics

Onions are part of the Allium family, which includes garlic, chives, leeks, and shallots (all of which can be used to substitute for onion if you run short). There are three basic forms of onion: white, yellow, and red (most sweet onions we see in stores, such as Vidalia, are of the yellow variety). Each has their optimal uses, with yellow being the most versatile for the kinds of cooking we do. Reds are the most likely to be used raw, particularly in salads, or when an accent color is desired. White is used more in white sauces or potato or pasta dishes where you want it to blend into the background.

The Science

Alliums have evolved with a special defense against animal's foraging on them in the wild. When the cells of an onion are ruptured, two enzymes and an amino acid are released. One of the enzymes, alliinase, biochemically splits the amino acid in two, releasing sulfenic acid which gives Alliums that sulfurous, pungent aroma and flavor. In onions, unlike the other Alliums, the second enzyme, lacrymatory factor synthase, then very quickly and almost completely transforms alliinase into lacrymatory factor, which just as quickly causes your eyes to water (see below for cutting techniques and other tips to minimize this).

Mincing an onion will bring more tears than chopping, and chopping more than slicing, because it's all about how many cells are ruptured, releasing the enzymes and amino acid. A sharp knife is obviously better for chopping onions (and everything else!) than a dull one for the same reason.

"It's hard to imagine civilization without onions."

Purchasing and Storing

Choose onions that are firm for their size, with skins that are still tightly adhering. Avoid any with cuts, bruises or blemishes. Larger onions will have more sulfurous compounds built up in its many layers, so lean toward smaller onions. Older onions are easier to peel but harder to hold together while slicing and chopping.

Store onions in a cool, dark, dry location with plenty of air movement. Cut onions can be stored in a sealed container in the fridge for up to 7 days, or in the freezer if the onion will subsequently be sautéed or cooked; i.e., not used raw.

Important: Onions and potatoes should not be stored together because onions produce ethylene gas which causes potatoes to sprout and spoil prematurely. Potatoes, for their part, give off moisture that can cause onions to go mushy. Storing them separately helps maintain their freshness and prevents spoilage. Both prefer air movement.

No More Tears

Before discussing slicing and chopping techniques, how to avoid tearing up when preparing onions? Two tips mentioned frequently both relate to chilling the onions before chopping: one by putting them in the freezer for 10-15 minutes, and the other by placing a peeled onion in cold water for a few minutes. And remember that smaller and fresher onions will be less sulfurous and less prone to creating lacrimatory factor.

Chopping Techniques

The classic onion chopping technique is done so that the root (the end with the little hairs) keeps the onion intact while you make your vertical and horizontal slices. This makes uniformity much easier because the onion doesn't fall apart in your hand as you chop. But it takes practice … and a sharp knife!

A diagram of the classic way to cut an onion while keeping control of the bulb and the size of the dice can be found in many resources. But it takes a sharp—and preferably thin—chef's knife blade to make the horizontal cut toward the root. That step takes some practice!

The Science Behind Chopping

Even the way you cut the onion affects its pungency. Slicing pole to pole (with the grain) ruptures fewer cells than slicing parallel to the equator (against the grain), and thus produces fewer of the sulfurous compounds. The older the onion, the stronger the compounds will be (and often harder to hold together while cutting).

Cutting an onion horizontally — across its width — makes for a quick way to soften your onion in cooking. Since the cell walls and fibers are broken, the onion cooks much faster. This cut is perfect when the onion is the backbone — not the star — of the show. The quick softening and flavor release make for a stronger taste, and it shrinks the size of the chopped pieces, great for integrating into sauces, soups, and purees.

"I will not move my army without onions!"

"Life is like an onion. You peel it off one layer at a time; and sometimes you weep."

How to Dice an Onion

1. Trim the top of the onion (opposite the root)
2. Slice the onion in half from end to end
3. Peel off the onion's skin. You can also remove the first layer if it feels tough
4. Lay the onion halves flat on the cutting board, make vertical cuts with the knife tip pointed at the root, stopping before you reach it. This keeps the onion from falling apart
5. Make horizontal cuts across the onion. Larger onions may need two or three cuts, while smaller onions might only need one
6. Make your final set of vertical cuts. The onion will dice into small cubes and tumble onto the cutting board