Melt butter
in large saucepan and add leeks and onion. Cook gently 5 minutes,
stirring. Do not brown. Add potatoes,
celery, parsley, chicken stock, salt, pepper, chervil and marjoram.
Cover and cook over moderate heat about 30 minutes or until vegetables
are tender. Whirl in the
blender (or press through a sieve). Return to saucepan. Bring back to a
boil and stir in hot milk. Taste and add more salt and pepper if
necessary.