Place the halved onion and ginger cut-side down in a dry pan over medium-high heat until charred and fragrant.
In a medium pot, lightly toast the star anise and cinnamon stick for 1 minute before adding the beef bone broth.
Add the charred onion, ginger, and fish sauce to the broth; bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, cook the rice stick noodles according to package directions, drain, and set aside.
Thinly slice the raw flank steak against the grain into very thin ribbons.
Strain the broth to remove the solids and return the clear liquid to the pot, seasoning with sea salt and black pepper.
Place the cooked noodles in a large bowl and top with the raw beef slices.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, cilantro, Thai basil, jalapeño, and a squeeze of lime.