A sharp knife and the right technique are all you need to cut an onion quickly, evenly, and without tears. Whether you need a fine dice for a soup base, thick rings for grilling, or a rough chop for a stew, the method stays the same with one small adjustment. This guide covers every cut you will ever need.
I used to hack at onions randomly until I watched Gordon Ramsay break one down in about 15 seconds on television. Once you understand the structure of an onion and work with the grain instead of against it, everything changes. It is faster, safer, and your food cooks more evenly because the pieces are the same size.
How to Hold an Onion Safely
Before any cutting, the most important thing is keeping your fingers safe. Use the claw grip: curl your fingertips under and press the flat of your knuckles against the side of the blade. Your knuckles guide the knife, and your fingertips never extend past them.
Always cut on a stable surface. If your chopping board slides, put a damp tea towel underneath it. A moving board is the fastest way to cut yourself in the kitchen.
How to Cut an Onion in Half
Every onion cut starts here. Slice the onion in half from root to tip (not through the equator). Leave the root end intact on both halves. The root holds the layers together while you cut, which is what gives you even, controlled pieces.
Peel the papery skin off each half. Place the flat, cut side down on your board. The onion is now stable and will not roll while you work.
How to Dice an Onion
Dicing gives you small, even cubes. This is the cut you need for soffritto, curry bases, bolognese sauce, soups, and any dish where the onion needs to melt into the background.
Step 1: Horizontal cuts. With the onion half flat on the board, make 2 to 3 horizontal cuts parallel to the board, slicing toward the root but stopping about 1cm before you reach it. Do not cut through the root. This step is optional for a rough dice but essential for a fine, even result.
Step 2: Vertical cuts. Make a series of vertical cuts from the top of the onion down toward the board, again stopping before the root. Space them about 5mm apart for a fine dice, or 1cm apart for a medium dice.
Step 3: Cross cuts. Now slice across the onion from the tip end toward the root. Even cubes will fall away as you cut. When you reach the root, discard it.
How to Chop an Onion
Chopping is a rougher, faster version of dicing. You skip the horizontal cuts entirely. Just make vertical cuts through the onion half, then slice across. The pieces will be less uniform but perfectly fine for stews, beef stew, casseroles, or any long-cooked dish where the onion breaks down anyway.
For a rough chop, space your vertical cuts about 1.5cm apart. For a finer chop, go closer to 5mm. The less precise you need to be, the faster this goes.
How to Slice an Onion
Slicing gives you long, curved pieces. There are two directions you can slice, and each gives a different result.
With the grain (pole to pole): Cut from root to tip, following the natural lines of the onion. These slices hold their shape during cooking and are perfect for caramelising, onion gravy, fajitas, and French onion soup. They soften but do not fall apart.
Against the grain (across the equator): Cut across the onion to create half-rings or rings. These break down faster and release more liquid. Good for quick-cooking dishes, salads, and burger toppings.
How to Cut Onion Rings
For perfect rings, do not cut the onion in half first. Peel the whole onion, then slice across it in even rounds. Push out the individual rings from each slice. Thick rings (about 1cm) work best for battering and frying. Thin rings are better for salads and pickles.
How to Mince an Onion
Mincing gives you the finest possible cut, almost a paste. Start by making a fine dice (see above), then run your knife back and forth across the pile using a rocking motion. Keep one hand on the spine of the blade and pivot it across the onion pieces until they are as fine as you need.
Minced onion is used in dressings, marinades, steak marinades, and anywhere you want onion flavour without visible pieces. You can also grate the onion on a box grater for a similar result.
How to Cut an Onion Without Crying
Onions release a chemical called syn-propanethial-S-oxide when their cells are damaged. It reacts with moisture in your eyes and produces a stinging sensation. Here is what actually works to reduce tears.
Use a sharp knife. A sharp blade crushes fewer cells, which means less gas is released. A dull knife does more damage to the onion structure and produces more of the irritant. This is the single biggest factor.
Chill the onion first. Put it in the fridge for 30 minutes before cutting. Cold temperatures slow down the chemical reaction and reduce how much gas is released.
Cut near a vent or open window. Moving air carries the gas away from your face. A fan pointed across your chopping board works too.
Cut the root end last. The root has the highest concentration of the enzyme that triggers tears. By leaving it intact as long as possible, you reduce exposure.
Which Onion to Use for What
Brown/yellow onion: The workhorse. Use it for cooking in soups, stews, gravies, sauces, and caramelising. It is the default choice for onion soup and most savoury dishes.
Red onion: Milder and slightly sweet when raw. Best for salads, salsas, pickles, and as a raw topping on burgers and sandwiches. It loses its colour when cooked, so use it where you want it to be seen.
White onion: Sharp and clean. Common in Mexican cooking and lighter dishes. Falls apart faster when cooked, so it is good for dishes where you want the onion to disappear into the sauce.
Shallot: Smaller, sweeter, and more refined. Use for dressings, pan sauces, and béarnaise sauce. Worth the extra cost when the onion is a primary flavour, not just a background ingredient.
Spring onion: Mild enough to eat raw. Slice finely for garnishing soups, scrambled eggs, noodles, and salads.
What Knife to Use
A chef’s knife (20 to 25cm blade) is the right tool for cutting onions. The curved blade allows for the rocking motion you need for mincing, and the length handles a full onion without cramping your technique.
A paring knife is too small for anything larger than a shallot. A serrated bread knife tears instead of slicing. Stick with a sharp chef’s knife and you will be faster and safer.
FAQs
Should you wash an onion after cutting?
Only if you are serving it raw and want a milder flavour. Rinsing diced or sliced raw onion under cold water removes some of the harsh bite. Pat dry before adding to a salad. Do not rinse if you are cooking the onion, the water just delays browning.
Can you prep onions ahead of time?
Yes. Store diced or sliced onion in an airtight container in the fridge for up to 3 days. The flavour gets slightly stronger over time, which is fine for cooking but not ideal for raw use. The container will smell strongly, so seal it well.
Why does the size of the cut matter?
Even-sized pieces cook at the same rate. If some pieces are large and others are tiny, the small ones burn while the large ones stay raw. Consistent cutting is one of the most practical skills you can develop in the kitchen.
What is the difference between dicing and chopping?
Dicing produces precise, even cubes using a three-cut technique (horizontal, vertical, cross). Chopping is rougher and faster, skipping the horizontal cuts. Dice when presentation or even cooking matters. Chop when the onion will cook down and nobody will see it.
Do you need to remove the root end?
Keep the root attached while cutting. It holds the layers together and gives you control. Only cut it off at the very end, and discard it. Removing the root first makes the onion fall apart and is harder to handle.
