Stir together beef broth, water, onion, garlic, and fish sauce (if using) in a large stockpot and bring to boil. Cover and reduce heat to simmer for about 15 minutes.
While beef broth is simmering, cook Pho noodles according to package directions or about 3 minutes till al dente. Drain and rinse with cold water to stop the cooking. Set aside.
After broth is finished, fill bowl about half-way with noodles, cover with thinly sliced steak, and ladle hot broth over the top. Steak will cook (if sliced thinly enough) when broth is poured over the top.