Trim and clean the leeks, by slicing them in half lengthways. Then slice them across in 1-cm/½-inch chunks. Melt the butter in a pan and add the leeks. Season, cover the pan and cook over a low heat for about 10 minutes. Stir and then blend to a purée. Add the Quark or cottage cheese and continue to blend until you have a smooth, thick sauce. Turn out on to a warm serving dish and sprinkle with the paprika.