How to Chop an Onion in a Food Processor

The metal chopping blade, the S-blade, is the only attachment that chops. A slicing disc makes rings and half-moons, not a chop, and a shredding disc tears onion into a ragged, watery pulp that is no substitute for chopping or for grated onion.

Peel the onion, trim both ends and cut the root nub away entirely. Knife work keeps the root to hold the onion together; in a processor the hard nub jams against the blade and produces splinters and uneven pieces. Start with a dry bowl, no water in it, and keep a spatula within reach for scraping down between pulses.

The one machine trait that matters most for this job is a responsive pulse button. A processor whose blade keeps coasting for a second or more after you release the button will over-chop onion no matter how careful your technique is.

1. Cut the peeled onion into even pieces. About 1-inch pieces for a full-size machine, 4 to 6 rough wedges for a 12- to 14-cup bowl, halves for a 3-cup mini chopper. The blade can only produce even pieces from even input, and a whole onion just bounces around the bowl.
2. Load the bowl no more than one-third to one-half full. For most full-size machines that means 1 to 2 medium onions, about 2 cups of pieces. This is the biggest single cause of uneven chopping: in an overfilled bowl the blade spins above the food and throws what it touches to the wall instead of circulating it, so you get a puddle of onion under the blade and chunks around the rim.
3. Seat the lid and pulse, about one second on and about one second off. Let the blade come to a complete stop between pulses. The pause lets gravity drop the pieces back into the blade's path; holding the button keeps the onion suspended and spinning, which bruises it and releases water.
4. Scrape down the bowl after every two or three pulses. Onion piles up on one side of the bowl, and redistributing it is what makes the chop even.
5. Count pulses by the texture you want. Inspect after the third pulse, because onion goes from chopped to pureed faster than any other vegetable.
6. Stop at the first sign of milky white liquid pooling at the bottom of the bowl. That liquid is cell fluid from ruptured cells; past that point the onion is bruised, wetter and heading for puree, and you cannot undo it.
7. Tip the onion into a strainer and drain it briefly if it looks wet. Wet onion steams in the pan instead of browning.
8. Wash the bowl, blade and lid right away in hot soapy water. Onion oils cling to plastic and set into a smell that lingers for days.

When in doubt, underprocess. You can always give it another pulse, and you can't go back.

The S-blade is the right tool, and everything on this page assumes it. The other attachments each do something different to an onion.

What a dicing kit really does to an onion

A dicing kit is the only attachment that produces true cubes, and it is the one attachment many people buy a processor for. Onion is also the hardest thing to put through a dicing grid: the pieces come out bruised and translucent at the corners rather than cleanly cut, and the grid clogs with onion, carrot and celery, which is why dicing kits ship with unclogging tools.

If you already own a dicing kit, use it for onion when you need cubes and accept the cleanup. If you are choosing a machine because you want to dice onions, don't let the dicing kit be the deciding feature: the S-blade plus good technique gets you a chop that cooks the same.

Every disappointing batch of processor onion traces back to one of a handful of causes, and each has a fix you can apply on the next onion.

"My onions came out as a watery puree"

The button was held down, the bowl was overfilled, or both. Onion is mostly water, and continuous blade contact ruptures the cells and releases it. Pulse in one-second bursts, stop between pulses, keep the bowl half full, and inspect after every pulse past the third.

"Some pieces are big, some are paste"

The input was uneven, the bowl was overfilled, or the onion was never scraped down. Cut the onion into even pieces first, fill the bowl less, and scrape down every two or three pulses.

"My onions taste bitter or harsh"

Over-processing is the cause. Damaging onion tissue sets off an enzymatic chain that produces sulfur compounds, including a family called allithiolanes, which taste bitter at very low concentrations. The bitterness isn't always there right away; it develops over about half an hour after the onion is cut. Chop coarser than you think you need, and cook the onion promptly rather than letting a finely chopped batch sit on the counter. If you are prepping ahead, leave the onion in larger pieces and chop it finer just before cooking.

"The bowl still smells days later"

Onion oils are oil-loving and cling to plastic. Wash the bowl, blade and lid immediately in hot soapy water and don't let it dry on. If the smell persists, soak the plastic parts for about 10 minutes in water with a splash of vinegar.

"The onion steamed instead of browning"

Excess water is the reason. Drain the chopped onion in a strainer before it hits the pan, and use a wide pan so the onion isn't crowded.

"It took forever and I kept stopping"

There was too much onion in the bowl. Split it into batches; two fast batches beat one slow, uneven one.

Chilling helps a little, for a different reason than most advice gives. A cold onion is firmer and cuts more cleanly, and cold slows how fast the irritant reaches your eyes. 10 to 15 minutes in the freezer is the practical window; longer and the outer layers start to freeze and the onion chops unevenly.

A food processor saves you the tears of dicing, but you still peel and cut the onion by hand, and that is where most of the crying happens. Cutting onion tissue releases an enzyme that turns onion compounds into a gas, syn-propanethial-S-oxide, which drifts up and irritates the eyes. Cold slows the gas forming; it doesn't stop it, and it doesn't switch the enzyme off.

Half an onion or one small onion isn't worth assembling, running and washing the machine; a knife is faster from start to finish. Raw onion that stars in the dish, in salsa, relish, vinaigrette or a garnish, is better knife-cut, because processor onion is bruised and leaks and you can taste and see the difference. A true fine brunoise is beyond the processor too, since the pieces won't be uniform enough, and grated onion for a marinade or meatball mix comes out better on a box grater than on a shredding disc.

The processor earns its place when you are chopping three or more onions, building a mirepoix, starting a ragu, bolognese, soup, stew or chili, making a curry paste, or meal-prepping for the week.

Mini machines chop onion well; they are just batch-limited. A 3-cup bowl takes about half a medium onion at a time, and the onion goes in as halves rather than quarters, because quarters leave too much empty space and the pieces bounce instead of meeting the blade.

Check after the second pulse rather than the third, since the blade sits close to the food in a small bowl. If the machine has a reversible blade with a Chop mode and a Puree mode, use Chop for onion: Chop mode leads with the flat back edge of the blade, which cuts more controllably, while Puree mode leads with the sharpened edge and gives a smoother, wetter result. Using the wrong mode is a common reason mini-chopper onions come out uneven.

Most mini processors and electric choppers have no discs at all, so there is no slicing and no shredding. If you want to slice onions as well as chop them, that is a full-size machine's job.

Pulse responsiveness comes first. The blade should stop quickly when you release the button, because a machine that coasts for a second or more after release will over-chop onion no matter how careful you are. This is the single most onion-relevant trait a processor can have.

A 14-cup bowl is great for three onions and bad for half of one, so if onions are most of what you chop, a 7- to 11-cup bowl is often the more useful size. A sharp, heavy S-blade matters too: a dull blade crushes onion instead of cutting it, which releases water and makes the mush problem worse.

A bowl that is easy to wash helps, since onion oils cling to plastic and a bowl and lid with fewer crevices and a removable blade hub are easier to get truly clean. A wide feed tube is worth having as well, for slicing onions with the disc and for dropping onion pieces through the running feed tube to mince them finely without over-processing the whole batch.

Chopped raw onion keeps about 3 to 4 days in a sealed container in the refrigerator. Raw chopped onion freezes well for up to about 8 months, and cooked onion dishes freeze for up to about 12 months.

Freeze it in a flat layer or in portions so you can break off what you need; frozen chopped onion goes straight into a hot pan. Frozen-then-thawed raw onion loses its crunch, so use frozen chopped onion for cooking rather than for raw applications.