This is a relatively simple cut as follows:

• Place the onion on a cutting board. Remove the stem end and trim the root, but leave it otherwise intact. Peel away the outer skin.
• Position the onion in the center of the board with the stem end facing your knife. Cut slices of your desired thickness.
• Stop when you reach the root end and discard it. You can now separate the onion slices into rings.

Tip: If you are not comfortable slicing an onion because it is round, hold it so the stem end is facing you, then cut away a quarter-inch piece from the side of the onion so it can lay flat on the cutting board.