technically…you do the whole thing.
After the gimme vision needed for story check off you get your mementos.
However…if you want to say lets not do the opening the doorway to the chamber bit that comes right after mother loses the cleanse option.
YOu have to open the door, do the fight in the chamber and then the cleanse option comes back.
At this point you are 99% done really. that last 1% is magni’s final run quest really.