Char the onion and ginger over an open flame or under a broiler until slightly blackened and fragrant.
In a large pot, bring the beef bone broth to a simmer with the charred onion, ginger, star anise, and cinnamon stick for 20 minutes.
While the broth simmers, prepare the rice noodles according to package directions until tender, then drain and set aside.
Thinly slice the raw beef against the grain into paper-thin pieces; freezing the beef for 20 minutes beforehand makes this easier.
Strain the broth to remove the solids, then stir in the fish sauce, sea salt, and black pepper.
Divide the cooked noodles into a large bowl and top with the raw beef slices.
Pour the boiling hot broth over the beef and noodles, which will cook the thin beef slices instantly.
Garnish with bean sprouts, fresh Thai basil, cilantro, and a squeeze of lime before serving.