Char the onion and ginger over an open flame or under a broiler until blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until the aroma is released.
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, cook the rice noodles according to package directions, then drain and set aside.
Slice the raw flank steak against the grain into paper-thin pieces.
Strain the solids from the broth and season with sea salt and black pepper.
Divide the noodles into bowls and top with the raw beef slices.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, basil, lime wedges, scallions, and jalapeño.