Traditionally, you spend a lot of effort on clear broth. You can, toward the end, add egg white only to remove it once it accumulates protein from the broth.

“name phở could have come from pot-au-feu”

Didn’t know that. The French also make consommé which is a very luxurious, very clear beef broth. I prefer Bun Bo Hu, but it’s not as easy to find good examples, even in my area.