How to Dice an Onion (Fast, Even, Fewer Tears)

To dice an onion, cut it in half through the root, peel it, make evenly spaced vertical cuts toward the root without cutting through it, then slice across those cuts. The root end holds the layers together so the onion falls into even cubes.

Dice sizes

Match the size to the cooking time. Small dice softens in 5 to 8 minutes of sweating; large dice survives a 3-hour braise without vanishing.

The science of onion tears

An intact onion does not smell much. Its cells store sulfur compounds (amino acid sulfoxides) separately from an enzyme called alliinase. When a knife ruptures the cells, the two meet. Alliinase produces sulfenic acids, and a second enzyme, lachrymatory factor synthase, converts them into syn-propanethial-S-oxide. That volatile gas drifts up, reacts with moisture on your eyes, and stimulates the nerves that trigger tears.

That chemistry explains every reliable trick:

• Sharp knife: a clean slice ruptures fewer cells than a crushing one, so less gas forms.
• Cold onion: chilling for 15 to 30 minutes slows the enzymes and lowers the gas's volatility. Do not freeze it; that ruins the texture.
• Leave the root on: the root end is generally thought to be the most pungent part, and keeping it intact also keeps cut layers from splaying open, so cut it last.
• Airflow: a fan or range hood moves the gas away before it reaches you.

Breathing through your mouth, holding bread in your teeth, and lighting a candle have little evidence behind them.

Steps

1. Chill the onion and sharpen your knife. A cold onion and a sharp blade mean fewer crushed cells and fewer tears.
2. Cut off the stem end, leaving the root intact. Slice about 1 cm off the pointed top. Trim only the hairy roots from the bottom, not the root plate itself.
3. Halve the onion through the root and peel it. Stand it on the cut top and slice down through the root. Peel off the papery skin and the first layer if it is leathery.
4. Lay each half flat and make vertical cuts toward the root. Point the knife tip toward the root and make parallel cuts from the root toward you, stopping about 1 cm short of the root. Space them at your dice size: 6 mm apart for small dice.
5. Add one or two horizontal cuts if the onion is large. Hold the knife parallel to the board and cut toward the root, again stopping short. Keep your palm flat on top and fingers out of the path.
6. Slice across the vertical cuts to release the dice. Use a claw grip and cut at the same spacing as your vertical cuts. Even cubes fall away.
7. Trim the root and chop the last piece. Lay the leftover root end flat and cut the remaining flesh, then discard the root or save it for stock.

Radial cuts for even dice

Onions are round, so parallel vertical cuts give slightly larger pieces at the outer edge. For more uniform dice, angle the vertical cuts like spokes of a wheel, pointing toward the centre of the onion beneath the board. It takes a little practice but produces very even pieces without horizontal cuts.

Chef Fizz says: The root is your onion's built-in clamp. Keep it on until the very last cut and the onion will dice itself into tidy little cubes.

Common mistakes and fixes

• Onion falls apart: you cut through the root plate. Leave the root intact until step 7.
• Big uneven chunks at the edges: use radial cuts, or make the outer vertical cuts slightly closer together.
• Strong, harsh flavour in raw dishes: cut onions release more pungent compounds over time. For salsa or salad, rinse the dice in cold water for a minute and drain, or soak in cold water for 10 minutes.
• Onion skids on the board: put a damp towel under the board and always cut with the flat side down.
• Mushy, wet onion: the knife is dull and crushing the cells. Sharpen it, and dice close to cooking time.

Storage

Whole dry onions keep for weeks in a cool, dark, ventilated spot away from potatoes. Diced onion keeps 7 to 10 days in an airtight container in the fridge, or up to 6 months frozen flat in a bag. Frozen onion softens, so use it for cooked dishes only.