Char the halved yellow onion and fresh ginger over an open gas flame or under a high broiler until blackened and fragrant.
In a large stockpot over medium heat, toast the star anise and cinnamon stick for 1 minute until aromatic.
Add the beef bone broth, charred onion, ginger, and tamari to the pot; bring to a boil, then reduce heat and simmer gently for 20 minutes.
While the broth simmers, cook the brown rice noodles according to package directions, drain, and set aside.
Place the flank steak in the freezer for 15 minutes to firm up, then slice against the grain into paper-thin strips.
Strain the broth through a fine-mesh sieve 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 serving bowl and arrange the raw steak slices on top.
Pour the boiling hot broth directly over the steak slices to cook them instantly, then garnish with bean sprouts, basil, cilantro, and a squeeze of lime.