Start by placing your onion on the cutting board. Trim off the top end of the onion (the pointy end) while leaving the root intact to hold the layers together.
Peel away the outer skin of the onion. Make a shallow cut into the skin to help remove it more easily. Discard the skin and any tough outer layers.
Place the onion cut-side down on the cutting board for stability. Slice it in half from the root to the cut end to get two halves.
Take one half of the onion and lay it flat on the cutting board. Holding it firmly, make horizontal cuts toward the root end without cutting all the way through. Make several cuts depending on desired dice size.
Turn the onion so the root is facing away from you. Make vertical cuts from top to root, again not cutting through the root. The spacing determines the size of the pieces.
Make perpendicular cuts across the previous slices to dice the onion evenly. Discard the root once done.
Repeat the process with the other half of the onion to have perfectly diced pieces ready for recipes.
Notes
Use a sharp knife to ensure clean cuts and reduce the chance of injury.
Keep the root intact while cutting to hold onion layers together for easier dicing.
Store leftover chopped onions in an airtight container in the refrigerator for up to one week or freeze for longer storage.