Official Weber Website
A round onion tends to move on a cutting board, which can be dangerous, so use one hand to hold it firmly in place. Use the other hand to cut straight down. You want thick, even slices.
Slices thinner than [conversion imperial="⅓ inch" metric="8 millimeters"] often fall apart on the grill, and slices that are not the same thickness from edge to edge cook unevenly and fall apart.