Char the onion and ginger in a dry pan over high heat until blackened in spots to release their deep, smoky aroma.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until fragrant.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot; bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, cook the brown rice noodles according to package directions, then drain and set aside.
Thinly slice the raw flank steak against the grain into very thin, translucent pieces.
Strain the broth into a clean pot, discarding the solids, and season with sea salt and black pepper.
Divide the cooked noodles into 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, Thai basil, scallions, and a squeeze of lime.