Place the halved onion and sliced ginger under a broiler for 5 minutes until charred and fragrant.
In a large pot over medium heat, toast the star anise and cinnamon stick for 2 minutes to release their essential oils.
Pour the beef bone broth into the pot and add the charred onion, ginger, and fish sauce.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to develop the aromatic profile.
While the broth simmers, cook the brown rice noodles according to package instructions, drain, and set aside.
Slice the raw flank steak against the grain into very thin, translucent strips.
Strain the broth through a fine-mesh sieve to remove solids, then return the liquid to the pot and season with sea salt and black pepper.
Place the cooked noodles and raw beef slices into a serving bowl.
Ladle the boiling hot broth directly over the beef to cook it instantly in the bowl.
Garnish with bean sprouts, Thai basil, jalapeño slices, and a fresh squeeze of lime before serving.