Buying, Storing, and Cooking Onions: A Guide

Published Categorized as Food
Onionsalexeykonovalenko /123RF

These alliums pack a fragrant and flavorful punch that can take the taste of your dishes to the next level. Here’s how to buy, store, and cook them.

Onions, the sweeter member of the allium family, is a staple ingredient in soups, stews, and chilis, and a can’t-miss add to American classics like burgers, hot dogs, and mac and cheese.

A versatile ingredient that can be eaten raw, pickled for that extra zing, sweated or sautéed in a hot skillet, boiled till soft and delicate, or roasted till browned and fall-apart tender, adding onions to your dish is an easy way to give it aroma and flavor.

Onions come in all colors, shapes, and forms—and knowing how to select them at the store and store them in your home is just as important as picking the right kind for your recipe.

The Types of Onions

Generally speaking, onions fall into five categories: sweet onions, red onions, white onions, yellow onions, and shallots. Each category has a unique aroma and flavor profile, and they can’t necessarily be used interchangeably.

Sweet onions are best for sweating, sautéing, and deep-frying; red onions for adding to salads and pickling; white onions for soups, stews, chilis, Mexican dishes, and British chutneys; yellow onions as an onion for all-around cooking; and shallots for casseroles, vinaigrettes, or as condiments on your burgers and hot dogs.

Cutting Onions

When you cut or crush onions, an enzyme called alliinase gets released and comes into contact with the sulfenic acid that’s also contained inside them, triggering a chemical reaction that forms a gas. That gas, much like tear gas, agitates your eyes and makes you cry.

Think of it as onions’ natural defense mechanism (a highly effective one, as a matter of fact) against predators. You can counter these defenses by chilling the onion. Submerge them in an ice bath for 5 minutes or throw them in your freezer for 10-15 minutes, and they won’t make you cry.

In addition to that, make sure your chef’s knife is razor-sharp. A dull blade will crush the onion instead of cutting into it, setting off more of its defense mechanisms as a result.

Cooking Onions

To get the most aroma and flavor out of your onions, slice them or dice them before cooking. Note that onions will hold their shape better when sliced lengthwise (stem to root) than crosswise.

To sweat onions, add a lump of butter or drizzle olive oil in your skillet generously, then preheat it for 2-3 minutes over medium-low heat. Once the frying pan is hot, add the onions and cook, stirring occasionally, for about 5 minutes so that they get wet and shiny.

To brown onions, coat the bottom of your skillet with olive oil and preheat for 2-3 minutes over medium-high heat. Add the onions and cook, stirring periodically, for 10 minutes until they’ve gotten golden-brown. Don’t use high heat as you will scorch the onions instead of brown them.

Do you cook onions or garlic first?

When a recipe calls for frying onions and garlic, cook the onions first, then add the garlic no longer than 20-30 seconds before you’re done sweating or browning them.

Once that time has passed, pour cooking liquid (water, sherry, wine, beer, broth) or throw the rest of the ingredients into your pan.

To roast onions, peel and halve them, toss with olive oil, then season abundantly with fine Himalayan or sea salt and freshly-cracked black pepper. Place them cut-side down on a sheet pan lined with parchment paper and roast for 30-45 minutes in a 375°F oven.

Choosing Onions at the Store

Every self-respecting grocery store carries onions in its produce section, usually unrefrigerated and close to other vegetables that grow under the ground, such as potatoes and beets.

Select onions at the grocery store by reaching for the ones that have intact peels, feel firm to the touch and heavy for their size, and seem free from cuts and blemishes. Sprouting and a developed seed steam are two tell-tale signs that an onion is dated—and that it might have gotten mushy on the inside.

Size does matter as far as onions are concerned. If a recipe calls for a small onion and you add a large onion instead, its taste will likely overpower all of the other ingredients in your dish.

As a general rule of thumb, small-sized onions are about the size of a clementine, medium-sized onions are closer to the size of a hefty lemon, and large-sized onions are comparable to oranges. Shallots are another story, as they’re much smaller than all other onions.

How to Store Onions

“The flavor of some hardier vegetables can be affected by the chilly fridge air,” Medical Doctor turned cookbook author Stuart Farrimond writes in his 2017 book, The Science of Cooking: Every Question Answered to Give You the Edge.

Farrimond recommends storing certain vegetables, such as potatoes, sweet potatoes, onions, and squash, in a cool, dark, well-ventilated place such as in a bin, big bowl, or large basket inside a kitchen cabinet or in your pantry. The reason behind that is simple: moisture, of which your fridge has plenty, encourages molding on onions.

If you prefer to refrigerate onions for one reason or another, store them at low humidity in the crisper drawer. Most fridges will let you control the humidity level with a dial that opens (low humidity) and closes (high humidity), revealing vent holes that promote air circulation.

Keep onions separated from potatoes, especially in your fridge. Onions and potatoes release moisture, so, stored together, they will spoil faster. Contrary to popular belief, neither produces ethylene gas, though they are both sensitive to it as it hastens their sprouting.

How Long Do Onions Last?

Kept in a cool, dark, well-ventilated place such as in a cabinet or in the pantry, raw, whole, unpeeled onions will last for 2-3 months from the day you unpacked them from your grocery bags.

Sliced or diced onions should be sealed in a ziplock bag or food storage container with the lid closed and kept in your fridge, where, as explained by Healthline, they will stay good for 10-14 days.

I stand by my assertion that wrapping onions in plastic wrap and aluminum foil are not great storage methods, as they lessen the onions’ shelf life and stink up your fridge.

Frozen onions stay safe to eat indefinitely. However, they will retain their best quality for 9-12 months. After that, they will start to dry out and lose flavor, becoming less and less appealing (even after they’re cooked) over time.

Thaw frozen onions by transferring them to your fridge the night before you plan to use them. Let them sit there overnight and, by the time you get cooking, they’ll be defrosted and workable once again.

An alternative (and faster) way to thaw onions is to seal them in a bag and submerge the bag in a large bowl filled with lukewarm water, changing the water every 10-15 minutes until the onions have fully defrosted.

By Jim Stonos

When Jim isn't in the kitchen, he is usually spending time with family and friends, and working with the HCW editorial team to answer the questions he used to ask himself back when he was learning the ropes of cooking.