Char the halved onion and sliced ginger under a broiler until the edges are blackened and fragrant.
In a large pot, combine the beef bone broth, charred onion, ginger, star anise, and cinnamon stick.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to infuse the flavors.
While the broth simmers, cook the rice noodles in a separate pot of boiling water according to package directions, then drain.
Season the broth with fish sauce, sea salt, and black pepper, then strain out the solids.
Return the clear broth to a rolling boil and prepare a serving bowl with the cooked noodles and raw, paper-thin beef slices.
Pour the boiling broth over the beef to cook it instantly and garnish with bean sprouts, fresh basil, and a squeeze of lime.