Place the halved onion and sliced ginger face down in a dry pan over medium-high heat until charred and aromatic.
In a large pot, toast the star anise and cinnamon stick for 1 minute until fragrant.
Add the beef broth, charred onion, ginger, and fish sauce to the pot; bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, place the flank steak in the freezer for 10 minutes to firm up, then slice it against the grain into paper-thin strips.
Cook the rice noodles according to package instructions in a separate pot, then drain and divide into serving bowls.
Strain the broth to remove the solids and season with sea salt and black pepper.
Place the raw beef slices over the noodles and pour the boiling hot broth directly over the meat to cook it instantly.
Top with bean sprouts, cilantro, Thai basil, and sliced green onions, serving with lime wedges on the side.