Thinly slice the flank steak against the grain while partially frozen for the best texture and set aside.
In a medium pot, char the onion and ginger over medium-high heat until slightly blackened and aromatic.
Pour in the beef bone broth, star anise, and cinnamon stick, then simmer for 15 minutes to infuse the flavors.
Cook the rice noodles according to package directions, drain, and place them in a large serving bowl.
Stir the fish sauce, sea salt, and black pepper into the hot broth and remove the whole spices.
Arrange the raw steak slices over the noodles and pour the boiling broth directly over the beef to cook it instantly.
Top the soup with bean sprouts, basil, cilantro, and a squeeze of lime before serving.