Cut off the ends of the green tops. Wash leeks well. Cut into 2-inch lengths and parboil 2 or 3 minutes. Drain.
Let the leeks in abaking dish. Dab each piece with butter and cook 10 minutes in a preheated 350° oven.
Serve in the leeks' own buttery juice with individual dishes of Hollandaise sauce for each person at the table.