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Knife Skills: How to Cut an Onion

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[Photographs: J. Kenji Lopez-Alt]

In the mood for some chili? You're gonna need three cups of onion, medium dice. Making chicken stock? Two onions, large chunks, please. And what about onion soup? Yes, believe it or not, you'll need onions for that too.

No matter how you slice 'em, onions are used in a good 30 to 40 percent of any cook's savory dish repertoire, if not more. They are the first thing you should learn how to cut when you pick up a knife, and, at least for me, are still one of the most pleasurable foods to take a sharp blade to.

In this guide, we'll go step-by-step through all of the basic onion cuts, as well as talking about the differences between various flavors of onions.

The Cuts

Quick tip: if you're working with a large volume of onions, to maximize efficiency, work by taking every onion through one step before proceeding to the next step. In other words, peel all of your onions before you start making horizontal slices on any of them. Similarly, make all of your horizontal cuts before making your vertical cuts. It will keep your work space more organized, require less trips to the garbage can (or compose can), and will make you look like a pro.

Shopping Guide

What color onion should you be using? For the most part, onions can be interchanged without overly catastrophic consequences (unless you consider red onions on a slider to be a catastrophe). But some onions are better suited for certain tasks than others.

The size of the onion has little bearing on flavor, though I prefer larger onions, only because it means I have to peel fewer of them to get the same volume of prepped onions.

No matter what onions you choose, make sure that they are firm to the touch when buying them. If they give even a little bit—particularly at the root or stem end—there's a good chance some of the interior layers may have begun to rot.

Store onions in a cool, dry, dark place. I keep mine inside a Chinese bamboo steamer, or sometimes hidden under my wife's side of the mattress if she's been getting on my nerves.

Now let's get down to business.

Peel off paper

Peel off the outer papery layers of skin by rubbing the onion firmly between your fingers until only the inner, tightly packed layers of skin remain.

Peeling this tough papery layer off will help prevent your knife from slipping later on down the line.

Trim stem end

Hold the onion steady with your non-knife and trim off the stem end by about 1/2-inch.

Slice in half

Lay the onion flat on its cut surface and slice it in half, using your non-knife hand to hold it steady.

Peel off outer layer

Peel off the remaining skin. The first pale layer underneath the skin can often be dry and tough, so it's a good idea to remove the outermost layer as well to reveal the more tender flesh underneath.

To Dice, Step 1: Cut along Z-axis

Lay the onion flat and make a series of horizontal slices, holding the top of the onion steady with the tips of your fingers. Slice nearly all the way through, but keep the root end intact so that layers remain connected.

Keeping the onion close to the edge of the board in order to give your knife hand clearance will facilitate this process.

To Dice, Step 2: Cut along Y-axis

Make a series of vertical cuts with the same spacing as your horizontal cuts, again keeping the root end intact. To hold the onion, curl back the tips of the fingers on your non-knife hand, keeping your thumb behind them in order to prevent accidentally cutting your fingertips or thumb. Hold the knife blade directly against your knuckles to guide your strokes.

To Dice, Step 3: Shift grips

Once you get close to the edge of the onion, use your non-knife hand to hold the onion steady by straddling it with your thumb and fingers.

To Dice, Step 4: Cut along X-axis

Finally, dice the onion by making a series of vertical cuts perpendicular to the ones you just made, again using the knuckles of your non-knife hand as a guide for the blade.

To Dice, Step 5: Transferring dice

Do not use your knife blade to pick up chopped vegetables—rubbing it against the cutting board will quickly dull its edge. Instead, use a bench scraper custom-designed for the task.

Fine and medium dice

The spacing of your horizontal and vertical cuts determines the size of your final dice. For large dice, make cuts 3/4 to 1-inch apart. For medium, about 1/2-inch. For fine dice, make cuts 1/4-inch or smaller, and for brunoise, cut as finely as possible—a very sharp knife and a steady hand should have no problem with 1/8th-inch or even 1/16th-inch cuts.

To slice into rings

To cut onion rings or half rings, simply peel the onion as for dicing, then cut parallel to the equator, using your knuckles as a guide. This cut is rarely used for cooking purposes, as an onion sliced parallel to the equator displays an undesirable wormy quality after cooking. For cooking applications, it's better to slice perpendicular to the equator.

To slice for cooking, Step 1: Trim root end

After trimming off the stem end and halving the onion, start by trimming 1/2 an inch off of the root end as well, then peeling off the outer layers.

To slice for cooking, Step 2: Slice

Make a series of slices perpendicular to the equator of the onion (pole-to-pole), once again using the knuckles of your non-knife hand as a guide.

To slice for cooking, Step 3

Continue slicing the entire onion. This is the cut you should use when a recipe calls for sliced onions. Onion slices cut pole to pole will break down more completely as they cook, producing a more uniform texture and flavor in the finished dish.

Sliced onions

Onions sliced pole-to-pole (left) and onions sliced parallel to the equator.

About the author: J. Kenji Lopez-Alt is the Chief Creative Officer of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.

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