Why leeks are good for you

The homely leek is a biddable, accommodating vegetable, but the cooking method is critical. An encounter with leek that's squeakily undercooked can be quite off-putting, and apart from soups, where the thinly sliced green flags add a characteristic whiff and bright colour, leeks are generally best kept away from water.
Fried leeks make a crisp, interesting garnish for bowls of Asian noodles or simple risottos. Cook them down slowly with butter and they swoon into a fondant state, while developing a savoury sweetness. At which point, they can bring another layer of flavour and a gooey thickness to ingredients such as ham, cheese, potato and pasta, or served as a bed of puree under grilled sausages and fish.
Why are leeks good for me? 
Leeks contain significant amounts of the flavonoid kaempferol. Many studies suggest it reduces the risk of developing chronic diseases – cancer in particular. It also appears to support our cardiovascular system by protecting our blood vessel linings.
Leeks are an excellent source of vitamin A, which aids vision and supports the immune system, and bone-building vitamin K and manganese. They also contain vitamin B6, which appears to reduce homocysteine in the blood; elevated levels of this amino acid are associated with a greater risk of heart disease, blood clots and strokes.
Source  - Guardian