Place the halved onion and sliced ginger under a broiler or over a gas flame until charred and aromatic.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until fragrant.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot and bring to a gentle simmer.
Let the broth simmer for at least 20 minutes to infuse the spices, then strain through a fine-mesh sieve and return the clear broth to a boil.
While the broth simmers, cook the rice sticks in boiling water according to package directions, then drain and divide into serving bowls.
Freeze the eye of round beef for 15 minutes to make it easier to slice into paper-thin rounds.
Arrange the raw, thin beef slices over the hot noodles in the bowls.
Ladle the boiling hot broth directly over the beef to cook it instantly to a tender medium-rare.
Serve immediately topped with bean sprouts, Thai basil, cilantro, sea salt, pepper, and a fresh lime wedge.