The quick answer
Halve a peeled onion through the root, place a half cut-side down and keep your fingers curled back. For dice, make lengthwise cuts toward the root without cutting it off, then cut across. For strips, slice from the stem side toward the root or follow the onion’s length for firmer pieces.
In this guide
Give the onion a stable base
Put a damp towel under a clean board if it slides. Use a sharp chef’s knife or a comfortable smaller blade; a blunt knife that needs force is harder to control. Trim the stem end, split the onion through its root and remove papery skin.
Set one half flat-side down. Keep your guiding fingertips curled inward, with knuckles directing the blade. Cut slowly enough to keep control. Do not hold the onion in the air or balance it on its curved outer surface.
Dice without a horizontal cut
Keep the root attached
The root end helps hold the layers together. Position it away from the knife’s starting point and leave a small uncut margin there.
Make parallel cuts
Slice lengthwise through the onion half, stopping before the root. Space cuts about 1/4 inch / 6 mm apart for a medium dice or wider for a rough chop. Keep the blade tip and fingers apart.
Cut across
Turn the board if that makes the movement easier. Cut across the first slices at similar spacing to release small pieces. A separate horizontal cut is not essential for everyday diced onion.
Finish safely
When the remaining end becomes too small to hold with a claw grip, lay it flat and chop carefully, or reserve it for stock. Discard the hard root base.
Choose slices for the cooking job
Crosswise slices make familiar half-moons. Lengthwise cuts from root toward stem produce strips that tend to keep their shape well in sautéed dishes. For fajitas, use fairly even strips so small pieces do not burn before thicker ones soften.
A fine dice disappears more readily into sauces; larger pieces suit roasting or soup. Consistent size matters more than a restaurant-perfect cube. Chop just before cooking when convenient, and refrigerate leftovers in a covered container.
About onion tears
A sharp blade and a calm pace reduce crushing. Ventilation and briefly chilling the onion may help some cooks, but no home trick guarantees tear-free cutting. Keep eyes and hands away from the blade while wiping your face; stop and set the knife down first.
A few useful answers
Must I make horizontal cuts to dice an onion?
No. For everyday cooking, lengthwise cuts followed by crosscuts give a useful dice without cutting sideways toward your hand.
What onion cut is best for fajitas?
Even lengthwise strips are convenient because they keep a recognizable shape as they soften.
Use the full recipe alongside these tips, and follow the stated care and safety limits of your equipment.
