This is one of the most useful techniques a cook can master. With a sharp knife and practice, you’ll be dicing like a pro in no time.
-Peel the onion, and cut it in half through the root.
-With the cut-side of the onion flat on the chopping board and the root-end pointing away from you, hold one side of the onion between your little finger and thumb. Place your other fingers on top of the onion.
-Angle the fingernail of your middle (or longest) finger and use that fingernail to guide and buffer the blade as it slices the onion very finely from one edge to the other. Meanwhile, with the hand holding the knife, ease the blade further up the onion with each cut. Do not slice the onion all the way through – leave 1cm at the root-end intact.
-Hold the onion at the root-end and, using a large sharp knife, make 4-5 horizontal cuts from the top of the onion to the root, stopping 1cm short of the root-end.
-Still holding the onion at the root-end, angle the fingernail of your middle (or longest) finger and use that fingernail to guide and buffer the blade as it slices the onion very finely from top to bottom. Meanwhile, with the hand holding the knife, ease the blade further up the onion with each cut. Discard the unchopped root-end; reserve the finely diced pieces of onion.
Source: https://www.bbc.co.uk
