Because the layers of plant cells in an onion run from the root to the stem end of the bulb, the direction in which you slice onions can make a big difference in their cooked appearance and texture.
When you slice across these layers, you separate them into small pieces that soften when cooked—ideal for pureeing into a smooth soup or sauce. However, when you slice with the direction of the fibers, you preserve their structure. That means the onions will maintain their shape even when cooked for a long time until deeply browned and flavorful, which makes them perfect for French onion soup or caramelized onions.
CROSSWISE TO SOFTEN
LENGTHWISE TO RETAIN SHAPE