Instructables
Picture of How to dice an onion
This is the simplest and quickest way to dice an onion. If you don't want to cry--just breathe through your mouth the entire time. If you don't breathe through your nose you should be fine.

**Disclaimer 1: This project involves sharp knives. If your knife is dull, you have a tendency to chop off your appendages, or you are small child, you should not attempt this project.**

**Disclaimer 2: If you don't follow the breathing instructions, and this makes you cry, make sure you wash your hands before rubbing your eyes...then find a shoulder to cry on...**
 
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Step 1: Slice off the top

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Slice off the top of the onion--do not cut off the root end!

Step 2: Peel off the skin

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Starting at the edge of the skin where you just cut, peel off the outer skin, down to the roots.

Step 3: Make 1st set of parallel cuts

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Holding the onion firmly, make parallel slices from the top towards the roots, without cutting completely through. Get as close to the roots as possible without detaching the section that you're cutting. Make the cuts as far apart as you want your diced pieces to be.

Step 4: Cut cross sections

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Hold the onion so that the slices you made stay together. Make the same series of slices perpendicular to your first set of cuts. From the top, your onion should look like a grid.

Step 5: Slice your heart out.

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Holding the onion together, lay it on its side, and carefully slice down. When you get close to the roots, you can lay it cut-side down and slice around the roots to use the entire onion.

Step 6: Voila!

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u2beal9997 years ago
Yes, in culinary school and in commercial kitchens, you always start by cutting the onion in half through the root, peel, then cut the majority of the root "hairs" while leaving the root intact. This makes for a cleaner work area. Also, by cutting the onion in half, the cut side can lay flat on the board. This allows you to put pressure, with a flat palm, on the top of the onion while making the initial horizontal slices. This way even if the knife slips you aren't in danger of cutting yourself because the blade is lower than your hand and facing away from the body.
gglockner7 years ago
Noooo! The onion is round, and you are making cuts towards your hand. This technique landed me in the hospital for five stitches. There IS a better way -- quarter the onion first, then you can keep the onion on flat sides for all the cuts. Much safer, and really no more work. I'll try to upload my own instructions when I get a chance. Thx.
dizzydave (author)  gglockner7 years ago
your quartering method sounds safer...i'll give it a shot next time...
Great instructions, and the comments are helpful too! I tend to do this near running water, and put the onion under the faucet from time to time when the fumes get too bad and I can't see.

I'm still getting used to the posting here, so forgive me if this isn't the place. I'll try to do an instructable on this at some stage. I wrote a short guide on choosing the right kind of knife. As the author points out, it's important to have a good sharp one.

Choosing A Cooking Knife

Cheers!
dizzydave (author)  Sir Chopsalot7 years ago
if you breathe through your mouth the entire time you're usually ok--but if you get that one whiff through your nose you're toast.
I cut it in half before I even take the skin off. I find it is easier to remove the skin and as previously mentioned the cut side prevents the onion from rolling about while cutting.
dizzydave (author)  RebelWithoutASauce7 years ago
I have taken to doing this as well...if I wasn't lazy I'd redo the instructable...
lemonie7 years ago
Not quite the classical technique, but modified as jtobako suggests by cutting in half exactly what I use.
You cannot appreciate how worthwhile this method is unless you've seen (or tried) mangling an onion in other ways (I have seen such, it amused me)

L
jtobako7 years ago
Cut the onion in half and do half at a time. That way, the onion won't roll your fingers into the knife blade : ) If you are horrified by slicing into the root, cut it off center or just a slice off one side for a sturdy base. After cutting the onion, rub your hands with something stainless steel under running water-it will get rid of the onion smell.
dizzydave (author)  jtobako7 years ago
hmm...that's a great idea! i've known lot's of people who do this trick, but no one ever cuts it in half...