Char the halved onion and sliced ginger in a dry pan over high heat until fragrant and slightly blackened on the edges.
In a large pot, combine beef bone broth, star anise, cinnamon stick, fish sauce, sea salt, and the charred aromatics.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to infuse the deep flavors.
While the broth simmers, cook the rice noodles according to package directions, drain, and divide into serving bowls.
Thinly slice the raw flank steak against the grain and arrange the slices over the warm noodles.
Strain the boiling hot broth and pour it directly over the beef to cook it instantly, then garnish with bean sprouts, Thai basil, black pepper, and lime wedges.