Peel off the papery outer skin
Trim the root end slightly, if needed
Cut the onion in half from top to root
Place each half flat-side down on a cutting board
Make vertical cuts toward the root, without cutting through it
Make horizontal cuts, if desired, for smaller pieces
Slice crosswise to dice
Separate the layers by hand if you want onion rings or petals
Rinse briefly under cold water if you want to reduce sharpness
Use immediately or store in an airtight container