Char the onion and ginger over an open flame or under a broiler until slightly blackened and aromatic.
In a large pot, toast the star anise and cinnamon stick over medium heat for 1-2 minutes until fragrant.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot; bring to a boil then simmer for 20 minutes.
While the broth simmers, soak the rice sticks in hot water according to package directions until tender, then drain.
Slice the raw flank steak against the grain into paper-thin pieces to ensure they cook quickly in the broth.
Strain the broth to remove the solids and return the clear liquid to a rolling boil.
Place the cooked noodles in a large bowl and top with the raw beef slices.
Pour the boiling broth over the beef to cook it instantly, then garnish with bean sprouts, cilantro, scallions, lime, salt, and pepper.