Cut the lower end of the bok choy. Set aside.
Slice the potato and onion. Set aside.
Meanwhile, pour water in a cooking pot. Apply heat.
Pour-in beef broth. Let boil.
Once the liquid starts to boil, stir-in onion and pork. Cover and simmer for 40 minutes or until pork gets tender. Add water or beef broth if needed.
Add the potato. Cover and cook for 8 minutes.
Pour-in the fish sauce and add the bok choy. Turn the heat off. Cover the cooking pot for 5 minutes.
Transfer to a serving bowl. Serve!