- 1 onion
A cutting board
A sharp, flat knife
Peel the onion and cut in half lengthwise, keeping the knife blade perpendicular to the cutting board.
Place the flat part of one-half of the onion on the cutting board. Keep the onion in place using one hand. Use the index (or middle) finger of the same hand to determine the thickness of each slice and to block the knife. Start slicing at one end of the half onion and make 1-2 mm slices by moving your fingers back a little at a time. The knife should remain in contact with the cutting board and, ideally, the tip of the knife should remain put. Finish slicing the second half of the onion.
Onions are used in many different recipes. When a recipe calls for a thickly sliced onion, the slice should be 1-2mm in thickness. A thick slice is about 3-4mm.