Char the halved onion and sliced ginger over an open flame or under a broiler until slightly blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until aromatic.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot; bring to a boil, then simmer for 20 minutes.
While the broth simmers, prepare the rice noodles according to package directions, then drain and set aside.
Place the flank steak in the freezer for 15 minutes to firm up, then slice it against the grain into paper-thin strips.
Strain the broth to remove the solids and return the clear liquid to the pot, seasoning with sea salt and black pepper.
Divide the cooked noodles into a large bowl and top with the raw beef slices.
Ladle the boiling hot broth over the beef and noodles to cook the meat instantly.
Garnish with bean sprouts, Thai basil, cilantro, and a squeeze of fresh lime juice before serving.