Qué is the question word. When what is not asking anything, it becomes lo que.
lo que = what (not a question)
Vi lo que haces · Cociné lo que comen · Lo que falta
No quiero que hables sobre lo que hablamos · No quiero que hables sobre lo que hablábamos
For about you can use de or sobre, and in the second half you need no about at all.
The pattern is a verb expressing what you want to happen in the world, followed by que.
Necesito que lo prepare · Necesito que usted lo prepare · Necesito que prepare lo que falta · Hace falta que comamos pronto · Prefiero que se quede · Prefiero que se queden · Prefiero que no se queden aquí
An awkward one to translate but very common. It is to be short of, to be missing, to lack.
Lo que falta (what's missing) · falta de consideración · falta de respeto
Hace falta has no comfortable literal translation, which is normal for fixed expressions. Take it as another way of saying it's necessary.
All the earlier machinery still applies. The se lo change and the a clarification work the same way here.
Prefiero que se lo des · Prefiero que se lo des a él · Prefiero que se lo des a ella
Necesito que lo prepare usted, when you need to show you mean formal you and not him or her.
A practical use for a choice that is otherwise a matter of feeling. Hablamos can be present or past, so a sentence built on it can be read either way.
Two fixes: add a time expression like el otro día, or pick the line past so the tense itself does the work. The meaning shifts so slightly that both are fine.