Hello Amara. This is one of the most common frustrations in home cooking, but with a few adjustments to your tools and technique, you can significantly reduce or even completely eliminate the tearing.
First and foremost, the sharpness of your knife is the single most important factor. When a blade is dull, it crushes the onion cells rather than slicing through them cleanly. This cell damage releases a much higher concentration of syn-propanethial-S-oxide, which is the volatile sulfur gas that reacts with the moisture in your eyes. A sharp chef's knife glides through cleanly with minimal cell breakage, dramatically cutting down the amount of irritant released into the air.
Second, leave the root intact until the very end. The basal plate at the bottom of the onion contains the highest density of these tear-inducing compounds. Cut the onion in half through the root, peel away the papery skin, and make your horizontal and vertical cuts while keeping the root end fully attached to anchor the layers. Only slice off and discard the root when you have finished dicing the rest.
Third, control your kitchen airflow. A simple and reliable method is to place your cutting board directly next to your stove and turn on the range hood exhaust fan on high. The fan draws the rising gases up and away from your face before they can reach your eyes.
Lastly, keep the cut sides facing down on your cutting board as you work. Minimizing the exposed surface area of the sliced onion helps prevent the vapors from dispersing into the room.
I hope these steps make your evening meal prep much more pleasant and tear-free.
