The main part of the leek that is eaten is the white to light green leaf sheaths. That long and dense part of the leek is not actually a stem or a stalk. They are leaves that are bound tightly together, giving them these gorgeous layers that make for a very pretty vegetable. The leafy green tops of the leek are extremely tough and are never really used independently as part of a dish. In most cases, they are discarded. However, they should be kept and used as a flavour enhancer. You can, for example, use them as the base of stock in your mirepoix just like you would with onions and in actual soups. You can tie them tightly together, submerge them in your soup and then discard them once the soup is ready. Like scallions, you can also use the leaves to form the base of meats and fish to help create flavour. Place them at the bottom of our baking dish or Dutch oven and place the meat on top.