The dark green leaves on leeks are extremely tough, so when incorporating leeks into a recipe, they are usually removed. But in some cases, they do have their uses, as the leaves are excellent for flavoring stocks.
When making soup at home, leeks can be used instead of a regular onion, or add them in addition for a little more flavor and texture. The Brie Soup with Leeks and Celery and Creamy Potato Leek Soup recipes below are just two examples.