Char the onion and ginger over an open flame or under a broiler until slightly blackened for deep flavor.
In a large pot, toast the star anise and cinnamon stick over medium heat until aromatic.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot and simmer for 20 minutes.
While the broth simmers, prepare the rice noodles according to package directions and set aside.
Place the flank steak in the freezer for 15 minutes to firm up, then slice it into paper-thin strips against the grain.
Strain the broth to remove the solids and return the liquid to a rolling boil, seasoning with sea salt and black pepper.
Place the cooked noodles and raw beef slices into a large serving bowl.
Pour the boiling broth over the beef to cook it instantly, then garnish with bean sprouts, cilantro, green onion, jalapeño, and a squeeze of lime.