To chop an onion by hand, peel it first. Then slice it in half vertically through the root, so that each half has part of the root in it.
This is the only time you cut through the root. No further cuts through the root — you want the root to be attached and intact up until the last minute, so that it can hold the onion together for you.
How large the chopped pieces you end up with depends on how thick you make your slices, so slice according to whether you want coarsely chopped, finely diced, etc.
Step 1 – slice the onion stop to bottom
Lay a half of the onion on its flat side, and make top to bottom cuts starting at the root end up to what was the top (not cutting through the root.)
Step 2 – slice the onion horizontally
Make horizontal slices from the top to the root (not cutting through the root.)
Step 3 – slice the onion sideways
Make sideways slices, with the handle of the knife towards you.
Start at the top of the onion, and make the slices, and the pieces of onion will fall off in a dice.
Slice through almost to the root end, then just discard the root end.
To chop onions in a food processor, use the blade that goes into the food processor bowl.
Peel and quarter the onions.
Pop a maximum of 15 to 20 quarters of onion at a time in, and whiz just until chopped.
You need to keep your eye on it, as it can go from chopped to puréed in the blink of an eye.
But if you’re chopping loads of onion for freezing, this is the only way to fly.