How to slice an onion

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The onion is an important basic ingredient with a wide variety of culinary uses and methods of preparation.  They have a distinctive bulbar shape surrounded by a delicate brittle skin that is yellow, red, or white.

yellow onions  red onions

They are available commonly as mature onion bulbs or in the immature pre-bulbing stage when they are known as scallions.


They come in different sizes, from jumbo softball sized, to regular tennis ball sized to pearl onions that are marble sized.

pearl onions  Shallots  Yellow onions

Regardless of the color, maturity, or size, all onions are made up of concentric rings that run from top to the root ends of the onion.  If you slice any onion horizontally (relative to the top and bottom root ends), you will see the concentric circles that are the meat of the onion by design.

sliced onion to see the rings

Have you ever wondered why onions cause so much eye irritation and tears?  Onions contain certain enzymes that catalyze a reaction to produce specific sulfur-containing acids.  (These enzymes are common to all plants in the same genus as the onion, such as garlic, leeks, scallions, and chives.)  When onions are cut, their cells are broken, and enzyme is released and free to produce this sulfur-containing acid.  This acid is then quickly transformed by another enzyme to make a volatile gas known as the lachrymatory factor, a name that implies watery eyes (since lachrymal glands are the tear-producing parts of the eyes).  This gas diffuses through the air easily, and when it reaches your eyes it activates sensory neurons that receive it as an irritation.  As a result, your eyes will naturally produce tears to try and flush out the irritant.

Many recipes call for thinly sliced onions.  This may feel like a chore because of the irritation that comes when dealing with onions.  And unfortunately, the onion chopper does not give you thin slices!  I used to dread working with onions because I hated them so much.  As I’ve mentioned in my previous post, I tried using all sorts of remedies to minimize eye irritation.  But because we effectively substitute onions with another ingredient or omit them altogether, we should learn proper ways to prepare them.  Thinly sliced onions are key ingredients to hearty meals, French onion soup, salads, sandwiches/burgers, and a variety of Chinese dishes.

I know there are quite a number of onion slicing methods out there, but I will share with you my method which has served me well over the years.

  1. Before you begin, be sure you have a good sized cutting board (preferably wooden) and a very sharp chef’s knife.  A sharp knife will allow you to cut an onion cleanly with faster strokes limiting the amount of juice that is squeeze out of the onion when cutting.
  2. Hold the chef’s knife by placing your thumb and index finger on the blade, and curling your other fingers firmly around the handle.  This is good practice whether you are left handed or right handed:
    hold chef's knife left hand  hold chef's knife right hand
  3. Practice fast and confidant cutting.  Use fennel bulbs at first.  They are shaped and layered like onions and they don’t give you eye irritation.  Practice makes perfect!
  4. Start by cutting the onion in half vertically through the center
    cut onion halfway vertically  onion cut in half vertically
  5. Cut off the top and root ends of the onion halves and peel the onion.  An onion is easier to peel once it is halved and the tops and roots are removed.
    cut off onion tops and roots
  6. Position one of the onion halves so that it is resting flat on the cutting board with the two cut ends pointing up and down (away from you and towards you, respectively).
  7. Hold the onion half with a firm grip pressing it against the cutting board.  Holding your knife at a low angle, your first slice will be lengthwise on one side of the onion.
    slice onion at a low angle
  8. Follow the natural curve of the onion by adjusting the angle of your knife as you slice.  The knife should be 90 degrees with the cutting board when you reach the middle of the onion half.
    follow onion's curve as you cut  knife is at 90 degrees at the middle
  9. Once you get to the middle, flip the onion half over and repeat.
  10. Then repeat steps 6 to 9 for the other half of the onion.  And then as they say, Bob’s your uncle!

I like this method because it allows for good control of the knife, and since the onion is halved it will not slip when you cut.  The onion slices turn out perfectly perpendicular to the onion’s natural surface without too much curvature as you would get if you cut them in cross-sections.  If you want thicker slices, simply adjust the angle of your knife as you make the next cut.


Related posts:

  1. Onion and vegetable chopper
  2. Roasted Kabochi and polenta tart with red onion stuffing

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