Cut the leeks in half lengthwise and then crosswise into thin slices. Place in a colander and run under warm water to remove all traces of sand. Drain and set aside. In a large casserole, melt the butter over low heat. Add the flour and cook, stirring constantly for one minute without browning. Add the chicken stock and whisk until well blended. Add the reserved leeks, carrots, turnips, parsnips and potatoes. Bring to a boil, reduce heat and simmer until the vegetables are almost tender. Add the lettuce and spinach leaves to the soup, and season with salt and pepper. The lettuce should just wilt, but still remain crisp. Do not overcook. Before serving, add the heavy cream and heat the soup through without letting it come to a boil. Transfer the soup to a large tureen, sprinkle with chives and parsley and serve hot with thin slices of black bread.