Peel away paper layer of onion, then use sharp kitchen knife to cut away onion top (stem end) and bottom (root end).
Place onion upright on cutting board, standing it on one of its now-flat ends. Slice onion into two equal halves, cutting down from top to bottom.
Peel away outer layer of onion halves.
Lay peeled onion half flat-side down on cutting board with one curved edge facing you. Make vertical slices along onion, about ½-inch apart. Slices should only go to approximately ¾- to 1-inch from edge of onion furthest from you, to keep it attached.
Rotate onion so one flat end is facing toward you, with attached end opposite your cutting hand. Following natural curve of onion, make angled cuts along onion's veins, roughly ½-inch apart. Cut completely through to center of onion, working your way around onion like a fan.
When you reach attached side of onion, make ½-inch wide slices in remaining piece of onion, positioning knife parallel to edge of onion.
Rotate slices onto their sides and make cuts every ½-inch to dice remaining onion.
Repeat process with other half of onion until completely diced. Break apart any pieces with multiple layers. Refrigerate diced onion in airtight container or use immediately.