Onions are a kitchen hero. They’re used in cuisines all over the world and are incredibly versatile. You can pickle them, caramelise them, serve ’em raw, and sauté them. Onions can serve as an aromatic base for a sauce, sit atop burgers, and even star on their own as a side dish, like these baked onions. Here, we break down everything you need to know about this allium, from the difference between storage onions and fresh onions, types of onions, to the best knife for cutting them.

The onion comes from the large lily family and the extensive AIlium genus. How can you identify one? If you crush it and you cry, it’s an allium. Sulfur compounds concentrated in the tissue of the onion are responsible for this reaction. Of the several hundred alliums in the world, most are wild species that have from time to time been gathered and eaten. The cultivated alliums — the ones sold in our markets — need no further introduction. They are onions, leeks, shallots, garlic, and chives. Onions make up a species called cepa, which also includes shallots.

Basically, there are two categories; the storage, or hot-onion, type and the fresh, or sweet-onion, type. (We’ve rated onions according to this heat index: hot/sharp, hot, moderately hot, and sweet/mild.)

Storage onions, which you find at the market year-round, do much of the seasoning work in kitchens around the world. They are sturdy, high in sulfur, low in water, and their taste ranges from moderately to hellishly hot. They have a natural resistance to moulds and bacteria. The grouping includes the following types of onions:

These are the most common cooking variety, and range from very small to moderately large. As strong and tear-provoking as they are, they lose their heat when cooked. This is an all-purpose onion — good in any raw or cooked dish and well-suited for stuffing because they don’t fall apart.

White storage onions have a sharp, crisp flavour. They have proportionately more water than the yellow onions, which makes them a little more perishable. Small to very large, whites taste sweet on first bite, but then the pungent sulfur flavour kicks in. They are great for sautéing and stewing.

Spanish onions are very large, round, yellow storage onions. Since they have a slightly higher water content, they are generally crisper, more perishable, and not so hot. Use them raw, sautéed, or caramelised.

Red onions fall on the moderately hot scale, and are similar to Spanish ones except for their hue. They also have a coarser flesh and are slightly sweeter, but still pungent. Like all onions, the larger ones are usually a tad less sharp and hold more moisture than their smaller siblings. They’re popular for pickling, and when pickled, make a punchy, acidic topping for dishes like tacos and chilli.

Boiling onions, about 1 1/4 to two inches (3 to 5 cm) in diameter, and pearl onions, about one to 1 1/4 inches (3 cm), are smaller versions of ordinary storage onions. There are some cultivars, or horticultural varieties, that are naturally small, but most boilers and pearls are created by deliberate crowding: farmers plant the fields so densely that the onions just don’t have room to grow. Pearls can be white, red, or gold. They are sweet, with a sharp bite — great for marinating or pickling. You can also use them in cocktails, like these Gibson Martinis (which feature homemade pickled white pearl onions). Boiling onions, white or yellow, are closer in flavour to large storage onions and just the right size for using whole in stews.

These variations on the storage onion include elongated purple onions such as Tropea or Florence long-red onions, and the intensely flavoured crisp, small, flat, button-shaped Cipollini onions. Cook and serve these whole, or cut them up and use in salads.

As previously mentioned, shallots are part of the same species as onions. While technically not categorised as a storage onion, they should be stored the same way and have a decently long shelf life (though not quite as long as a storage onion), so we’ve lumped them in here. They have a more mild and sweet taste than storage onions, and grow in clusters. There are different varieties, including pink shallots, grey shallots, and more. Fry them and use them as a delicious topping, turn them into Caramelised Shallot Sambal Bawang, or add them to a tart.

This is the second major category of onions, encompassing sweet onions like Vidalia onions, and other fresh onions, like scallions and ramps (we’ve included leeks in here as well). These onions are not keepers and are meant to be used quickly. The grouping includes the following types of onions:

Sweet onions are most often sold under a regional name, such as Vidalia (Georgia), Maui (Hawaii), and Walla Walla (Washington), and there is a parade of them at markets through the spring and summer. They generally have perfectly concentric centres, which make them ideal for onion rings, and because of their high sugar content, they caramelise easily. They’re also great on burgers and in salads.

Scallions, which are sometimes called green onions, are harvested while young and slender and marketed with the green tops still intact. They are generally considered a subcategory of fresh onions, and are good raw and cooked. The green tops and the white bulbs are eaten.

Spring onions have a white bulb at their base and green tops like scallions. They have a mild taste, and can be used in dishes like fresh salads — like this Asparagus and Spring Onion Salad with Seven-Minute Eggs — and this Carrot and Spring Onion Toad in the Hole. You can also try grilling them. They’re an early version of a storage onion that, if left in the ground, would grow to a full onion.

Ramps are a wild onion native to eastern North America that are a prized ingredient among chefs, and have a famously short season in the spring. They have a small white bulb on the bottom, a thin stalk that can have a mix of pink and white colours, and wide and flat green leaves on top. The flavour is pungent, and we’ve found that the leaves have a garlicky flavour while the bulbs taste oniony. You can pickle them, turn the leaves into a powder, add them to pasta, and more.

Leeks are thicker and longer than other fresh onions, with a white base that turns darker green the further you go up the stalk (only use the white and light green parts). They have a very mild flavour, and we like to use them in everything from gratins to soup. They tend to be sandy, so you’ll need to clean them well before using. We recommend either halving them lengthwise and cleaning them under running cool water in the sink, or slicing them and soaking them in cold water so the sand can sink to the bottom of the container.

According to the National Onion Association (NOA), when selecting dry bulb onions, you want them to be firm, with “little to no scent.” They should not have grey or black mould on them, and you should skip ones that show wear and tear, like bruises. They’ll keep for many months in a cool, dark, dry place with good air circulation — avoid storing them in containers like plastic bags, which won’t keep them well-ventilated. Once you’ve cut the onions, they can be kept in the fridge in an airtight container for up to a week. Shallots should also be kept in a cool, dark, and dry place, and can keep for several weeks.

As for fresh onions, storage and shelf life vary. For ramps, we recommend keeping them in the fridge, in a jar of water or wrapped in a damp dish towel in your crisper drawer, and they should last for a few days. Leeks, per Allrecipes, are good in the fridge for up to two weeks (keep ’em loosely wrapped in plastic wrap). If you’ve got scallions, The Spruce Eats recommends two storage options — in the crisper drawer wrapped in damp paper towels for a few days, or in a glass jar with water (plus a plastic produce bag) for up to a week. And spring onions, another crisper drawer candidate, can be stored in a plastic bag in there for a week or two.

If you have sweet onions, per The Spruce Eats, these will last in the fridge for a maximum of a few weeks (wrap them individually in a paper towel for the best results). In general, when storing sweet and mild onions in the fridge, the NOA recommends using a low humidity setting.

When cutting onions, use a stainless steel knife, since a carbon steel blade can sometimes discolour the onions. There is a greater concentration of the irritating sulfur compound in the root, so always cut from the top (the neck) to the bottom in order to leave the root end intact for as long as possible. To peel pearl and boiling onions, blanch them in a pot of boiling water for one minute, then drain and plunge them into ice water. Using a small sharp knife, cut off both the root and stem ends and remove the skin. Only chop onions in a food processor when you are using them in a soup or a very runny sauce, because this will bruise the pieces and cause them to disintegrate when cooked.

If you find your onion has sprouted — aka, that there’s a green stalk poking out the top — have no fear. You can still eat them, but cut the sprout out before using the onion, since it’s typically bitter. You’ll also want to use that onion quickly, since the sprout is an indicator that the texture and flavour are going downhill.

There’s a lot you can do with onions. If you have red onions, try using them in a dish like this Sausage and Red Onion Sheet Pan Quiche, or Grilled Chicken with Marinated Tomatoes and Onions. Pearl or cipollini onions can be used in this rich, warming Beef Stew in Red Wine Sauce from chef Jacques Pépin. Vidalia onions, on the other hand, lend themselves to this Roasted Sweet Onion Dressing, paired with a Southern Cobb Salad, and yellow onions are one of the stars in this recipe for Stir-Fried Flank Steak with Yellow Onions. Have a few different types of onions on hand? This Caramelised Five-Onion Dip is your friend since it calls for a large sweet onion, a yellow onion, a red onion, shallots, and scallions for a deeply flavorful appetiser. And if you want to know how to caramelise onions, we have a step-by-step guide and recipe, which calls for equal parts red and yellow onions — the sweet, jammy results will be well worth the wait.

This story first appeared on www.foodandwine.com

(Credit for the hero and featured image: Sveta / Getty Images)

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