Ensure you have a clean green chopping board prepared. Cut the onion in half lengthwise using a medium knife. Cut 5mm slices in onion lenghthwise towards the root but not through the root. Slice horizontally every 5mm towards the root but not through the root of the onion. Slice 5mm across the onion to form small dice. Keep root end for stock. Place the small dice onion in a plastic container with a cover. Place in refrigerator until required.