How Slicing Impacts Onions
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.