Char the onion and ginger over an open flame or under a broiler until the skins are blackened and aromatic.
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 the heat and simmer for 20 minutes to allow the spices to infuse deeply.
While the broth is simmering, cook the rice stick noodles according to the package directions, then drain and set aside.
Stir the fish sauce, sea salt, and black pepper into the broth; remove and discard the whole spices, onion, and ginger.
Place the cooked noodles in a large serving bowl and arrange the raw, paper-thin beef slices on top.
Pour the boiling hot broth directly over the beef slices, which will cook them instantly until they are just tender.
Garnish the soup with bean sprouts, cilantro, and Thai basil, and serve immediately with a fresh lime wedge.