Step 1: The Tools
To prep an onion, start with a very sharp chef's knife or santoku knife and a large cutting board to help prevent runaway pieces. A good bench scraper will facilitate transferring the cut onion to your prep bowl or pan.
Step 2: Peel off paper
Peel off the outer papery layers of skin by rubbing the onion firmly between your fingers until only the inner, tightly packed layers of skin remain.
Peeling this tough papery layer off will help prevent your knife from slipping later on down the line.
Step 3: Trim stem end
Hold the onion steady with your non-knife and trim off the stem end by about 1/2-inch.
Step 4: Slice in half
Lay the onion flat on its cut surface and slice it in half, using your non-knife hand to hold it steady.
Step 5: Peel off outer layer
Peel off the remaining skin. The first pale layer underneath the skin can often be dry and tough, so it's a good idea to remove the outermost layer as well to reveal the more tender flesh underneath.
To Dice, Step 1: Cut along Z-axis
Lay the onion flat and make a series of horizontal slices, holding the top of the onion steady with the tips of your fingers. Slice nearly all the way through, but keep the root end intact so that layers remain connected.
Keeping the onion close to the edge of the board in order to give your knife hand clearance will facilitate this process.
To Dice, Step 2: Cut along Y-axis
Make a series of vertical cuts with the same spacing as your horizontal cuts, again keeping the root end intact. To hold the onion, curl back the tips of the fingers on your non-knife hand, keeping your thumb behind them in order to prevent accidentally cutting your fingertips or thumb. Hold the knife blade directly against your knuckles to guide your strokes.
To Dice, Step 3: Shift grips
Once you get close to the edge of the onion, use your non-knife hand to hold the onion steady by straddling it with your thumb and fingers.
To Dice, Step 4: Cut along X-axis
Finally, dice the onion by making a series of vertical cuts perpendicular to the ones you just made, again using the knuckles of your non-knife hand as a guide for the blade.
To Dice, Step 5: Transferring dice
Do not use your knife blade to pick up chopped vegetables—rubbing it against the cutting board will quickly dull its edge. Instead, use a bench scraper custom-designed for the task.
Fine and medium dice
The spacing of your horizontal and vertical cuts determines the size of your final dice. For large dice, make cuts 3/4 to 1-inch apart. For medium, about 1/2-inch. For fine dice, make cuts 1/4-inch or smaller, and for brunoise, cut as finely as possible—a very sharp knife and a steady hand should have no problem with 1/8th-inch or even 1/16th-inch cuts.
To slice into rings
To cut onion rings or half rings, simply peel the onion as for dicing, then cut parallel to the equator, using your knuckles as a guide. This cut is rarely used for cooking purposes, as an onion sliced parallel to the equator displays an undesirable wormy quality after cooking. For cooking applications, it's better to slice perpendicular to the equator.
To slice for cooking, Step 1: Trim root end
After trimming off the stem end and halving the onion, start by trimming 1/2 an inch off of the root end as well, then peeling off the outer layers.
To slice for cooking, Step 2: Slice
Make a series of slices perpendicular to the equator of the onion (pole-to-pole), once again using the knuckles of your non-knife hand as a guide.
To slice for cooking
Continue slicing the entire onion. This is the cut you should use when a recipe calls for sliced onions. Onion slices cut pole to pole will break down more completely as they cook, producing a more uniform texture and flavor in the finished dish.
Onions sliced pole-to-pole (left) and onions sliced parallel to the equator.