Leeks are considered the holy grail of onions by some folks. When sautéed, leeks melt into sweet nothingness and serve as a superior base to any soup or stock: potato leek soup or vichyssoise, chicken noodle soup, vegetable or chicken stock, etc. Thinly sliced, you can also use the whites and light green parts raw in salads.
Take care to clean your leeks well before cooking with them, you’ll find dirt between the leaf sheathes.